Methods of Study in Natural History

ALL important changes in the social and political condition of man, whether brought about by violent convulsions or effected gradually, are at once recognized as eras in the history of humanity. But on the broad high-road of civilization along which men are ever marching, they pass by unnoticed the landmarks of intellectual progress, unless they chance to have some direct bearing on what is called the practical side of life. Such an era marked the early part of our own century; and though at the time a thousand events seemed more full-freighted for the world than the discovery of some old bones at the quarry of Montmartre, and though many a man seemed greater in the estimation of the hour than the professor at the Jardin des Plantes who strove to reconstruct these fragments, yet the story that they told lighted up all the past, and showed its true connection with the present. Cuvier, as one sees him in a retrospective glance at the wonderful period in which he lived, and which brought to the surface all its greatest elements, — one among a throng of exceptional men, generals, soldiers, statesmen, as well as men of commanding intellect in literary and scientific pursuits, — seems always standing at the meeting-point between the past and present. His gaze is ever fixed upon the path along which Creation has moved, and, as he travels back, recovering step by step the road that has been lost to man in apparently impenetrable darkness and mystery, the light brightens and broadens before him, and seems to tempt him on into the dim regions where the great mystery of Creation lies hidden.

Before the year 1800, men had never suspected that their home had been tenanted in past times by a set of beings totally different from those that inhabit it now; still farther was it from their thought to imagine that creation after creation had followed each other in successive ages, every one stamped with a character peculiarly its own. It was Cuvier who, aroused to new labors by the hint he received from the bones unearthed at Montmartre, to which all his vast knowledge of living animals gave him no clue, established by means of most laborious investigations the astounding conclusion, that, prior to the existence of the animals and plants now living, this globe had been the theatre of another set of beings, every trace of whom had vanished from the face of the earth. To his alert and active intellect and powerful imagination a word spoken out of the past was pregnant with meaning; and when he had once convinced himself that he had found a single animal that had no counterpart among living beings, it gave him the key to many mysteries.

It may be doubted whether men’s eyes are ever opened to truths which, though new to them, are old to God, till the time has come when they can apprehend their meaning and turn them to good account. It certainly seems, that, when such a revelation has once been made, light pours in upon it from every side; and this is especially true of the case in point. The existence of a past creation once suggested, confirmation was found in a thousand facts overlooked before. The solid crust of the earth gave up its dead, and from the snows of Siberia, from the soil of Italy, from caves of Central Europe, from mines, from the rent sides of mountains and from their highest peaks, from the coral beds of ancient oceans, the varied animals that had possessed the earth ages before man was created spoke to us of the past.

No sooner were these facts established, than the relation between the extinct world and the world of to-day became the subject of extensive researches and comparisons; innumerable theories were started to account for the differences, and to determine the periods and manner of the change. It is not my intention to enter now at any length upon the subject of geological succession, though I hope to return to it hereafter in a series of papers upon that and kindred topics ; but I allude to it here, before presenting some views upon the maintenance of organic types as they exist in our own period, for the following reason. Since it has been shown that from the beginning of Creation till the present time the physical history of the world has been divided into a succession of distinct periods, each one accompanied by its characteristic animals and plants, so that our own epoch is only the closing one in the long procession of the ages, naturalists have been constantly striving to find the connecting link between them all, and to prove that each such creation has been a normal and natural growth out of the preceding one. With this aim they have tried to adapt the phenomena of reproduction among animals to the problem of creation, and to make the beginning of life in the individual solve that great mystery of the beginning of life in the world. In other words, they have endeavored to show that the fact of successive generations is analogous to that of successive creations, and that the processes by which animals, once created, are maintained unchanged daring the period to which they belong will account also for their primitive existence.

I wish, at the outset, to forestall any such misapplication of the facts I am about to state, and to impress upon my readers the difference between these two subjects of inquiry,—since it by no means follows, that, because individuals are endowed with the power of reproducing and perpetuating their kind, they are in any sense self-originating. Still less probable does this appear, when we consider, that,since man has existed upon the earth, no appreciable change has taken place in the animal or vegetable world; and so far as our knowledge goes, this would seem to be equally true of all the periods preceding ours, each one maintaining unbroken to its close the organic character impressed upon it at the beginning.

The question I propose to consider here is simply the mode by which organic types are preserved as they exist at present. Every one has a summary answer to this question in the statement, that all these short-lived individuals reproduce themselves, and thus maintain their kinds. But the modes of reproduction are so varied, the changes some animals undergo during their growth so extraordinary, the phenomena accompanying these changes so startling, that, in the pursuit of the subject, a new and independent science — that of Embryology — has grown up, of the utmost importance in the present state of our knowledge.

The prevalent ideas respecting the reproduction of animals are made up from the daily observation of those immediately about us in the barn-yard and the farm. But the phenomena here are comparatively simple, and easily traced. The moment we extend our observations beyond our cattle and fowls, and enter upon a wider field of investigation, we are met by the most startling facts. Not the least baffling of these are the disproportionate numbers of males and females in certain kinds of animals, their unequal development, as well as the extraordinary difference between the sexes among certain species, so that they seem as distinct from each other as if they belonged to separate groups of the Animal Kingdom. We have close at hand one of the most striking instances of disproportionate numbers in the household of the Bee, with its one fertile female charged with the perpetuation of the whole community, while her innumerable sterile sisterhood, amid a few hundred drones, work for its support in other ways. Another most interesting chapter connected wuth the maintenance of animals is found in the various ways and different degrees of care with which they provide for their progeny: some having fulfilled their whole duty toward their offspring when they have given them birth ; others seeking hidingplaces for the eggs they have laid, and watching with a certain care over their development; others feeding their young till they can provide for themselves, and building nests, or burrowing holes in the ground, or constructing earth mounds for their shelter.

But, whatever be the difference in the outward appearance or the habits of animals, one thing is common to them all without exception: at some period of their lives they produce eggs, which, being fertilized, give rise to beings of the same kind as the parent. This mode of generation is universal, and is based upon that harmonious antagonism between the sexes, that contrast between the male and the female element, that at once divides and unites the whole Animal Kingdom. And although this exchange of influence is not kept up by an equality of numeric relations, — since not only are the sexes very unequally divided in some kinds of animals, but the male and female elements are even combined in certain types, so that the individuals are uniformly hermaphrodites,— yet I firmly believe that this numerical distribution, however unequal it may seem to us, is not without its ordained accuracy and balance. He who has assigned its place to every leaf in the thickest forest, according to an arithmetical law which prescribes to each its allotted share of room on the branch where it grows, will not have distributed animal life with less care.

But although reproduction by eggs is common to all animals, it is only one among several modes of multiplication. We have seen that certain animals, besides the ordinary process of generation, also increase their number naturally and constantly by self-division, so that out of one individual many individuals may arise by a natural breaking up of the whole body into distinct surviving parts. This process of normal self-division may take place at all periods of life : it may form an early phase of metamorphosis, as in the Hydroid of our common Aurelia, described in the last article; or it may even take place before the young is formed in the egg. In such a case, the egg itself divides into a number of portions: two, four, eight, or even twelve and sixteen individuals being normally developed from every egg, in consequence of this singular process of segmentation of the yolk, which takes place, indeed, in all eggs, but in those which produce but one individual is only a stage in the natural growth of the yolk during its transformation into a young embryo. As the facts here alluded to are not very familiar even to professional naturalists, I may be permitted to describe them more in detail.

No one who has often walked across a sand-beach in summer can have failed to remark what the children call “sand saucers.” Tie name is not a bad one, with the exception that the saucer lacks a bottom ; but the form of these circular bands of sand is certainly very like a saucer with the bottom knocked out. Hold one of them against the light and you will see that it is composed of countless transparent spheres, each of the size of a small pin’s headThese are the eggs of our common Natica or Sea-Snail. Any one who remembers the outline of this shell will easily understand the process by which its eggs are left lying on the beach in the form I have described. They are laid in the shape of a broad, short ribbon, pressed between the mantle and the shell, and, passing out, cover the outside of the shell, over which they are rolled up, with a kind of glutinous envelope, — for the eggs are held together by a soft glutinous substance. Thus surrounded, the shell, by its natural movements along the beach, soon collects the sand upon it, the particles of which in contact with the glutinous substance of the eggs quickly forms a cement that binds the whole together in a kind of paste. When consolidated, it drops off from the shell, having taken the mould of its form, as it were, and retaining the curve which distinguishes the outline of the Natica. Although these saucers look perfectly round, it will be found that the edges are not soldered together, but are simply lapped one over the other. Every one of the thousand little spheres crowded into such a circle of sand contains an egg. If we follow the development of these eggs, we shall presently find that each one divides into two halves, these again dividing to make four portions, then the four breaking up into eight, and so on, till we may have the yolks divided into no less than sixteen distinct parts. Thus far this process of segmentation is similar to that of the egg in other animals ; but, as we shall see hereafter, it seems usually to result only in a change in the quality of its substance, for the portions coalesce again to form one mass, from which a new individual is finally sketched out, at first as a simple embryo, and gradually undergoing all the changes peculiar to its kind, till a new-born animal escapes from the egg. But in the case of the Natica this regular segmentation changes its character, and at a certain period, in a more or less advanced stage of the segmentation, according to the species, each portion of the yolk assumes an individuality of its own, and, instead of uniting again with the rest, begins to subdivide for itself. In our Natica heros, for instance, the common large gray Sea-Snail of our coast, this change takes place when the yolk has subdivided into eight parts. At that time each portion begins a life of its own, not reuniting with its seven twin portions; so that in the end, instead of a single embryo growing out of this yolk, we have eight embryos arising from a single yolk, each one of which undergoes a series of developments similar in all respects to that by which a single embryo is formed from each egg in other animals. We have other Naticas in which the normal number is twelve, others again in which no less than sixteen individuals arise from one yolk. But this process of segmentation, though in these animals it leads to such a multiplication of individuals, is exactly the same as that discovered by K. E. von Baer in the egg of the Frog, and described and figured by Professor Bischof in the egg of the Rabbit, the Dog, the Guinea-Pig, and the Deer, while other embryologists have traced the same process in Birds, Reptiles, and Fishes, as well as in a variety of Articulates, Mollusks, and Radiates.

Multiplication by division occurs also normally in adult animals that have completed their growth. This is especially frequent among Worms; and strange to say, there are species in this Class which never lay eggs before they have already multiplied themselves by self-division.

Another mode of increase is that by budding, as in the Corals and many other Radiates. The most common instance of budding we do not, however, generally associate with this mode of multiplication in the Animal Kingdom, because we are so little accustomed to compare and generalize upon phenomena that we do not see to be directly connected with one another. I allude here to the budding of trees, which year after year enlarge by the addition of new individuals arising from buds. I trust that the usual acceptation of the word individual, used in science simply to designate singleness of existence, will not obscure a correct appreciation of the true relation of buds to their parents and to the beings arising from them. These buds have the same organic significance, whether they drop from the parent stock to become distinct individuals in the common acceptation of the term, or remain connected with the parent stock, as in Corals and in trees, thus forming growing communities of combined individuals. Nor will it matter much iu connection with the subject under discussion, whether these buds start from the surface of an animal or sprout in its interior, to be cast off in due time. Neither is the inequality of buds, varying more or less among themselves, any sound reason for overlooking their essential identity of structure. We have seen instances of this among Acalephs, and it is still more apparent among trees which produce simultaneously leaf and flowerbuds, and even separate male and female flower-buds, as is the case with our Hazels, Oaks, etc.

It is not, however, my purpose here to describe the various modes of reproduction and multiplication among animals and plants, nor to discuss the merits of the different opinions respecting their numeric increase, according to which some persons hold that all tvpes originated from a few primitive individuals, while others believe that the very numbers now in existence are part of the primitive plan, and essential to the harmonious relations existing between the animal and vegetable world. I would only attempt to show that in the plan of Creation the maintenance of types has been secured through a variety of means, but under such limitations, that, within a narrow range of individual differences, all representatives of one kind of animals agree with one another, whether derived from eggs, or produced by natural division, or by budding; and that the constancy of these normal processes of reproduction, as well as the uniformity of their results, precludes the idea that the specific differences among animals have been produced by the very means that secure their permanence, of type. The statement itself implies a contradiction, for it tells us that the same influences prevent and produce change in the condition of the Animal Kingdom. Facts are all against it; there is not a fact known to science by which any single being, in the natural process of reproduction and multiplication, has diverged from the course natural to its kind, or in which a single kind has been transformed into any other. But this once established, and setting aside the idea that Embryology is to explain to us the origin as well as the maintenance of life, it yet has most important lessons for us, and the field it covers is constantly enlarging as the study is pursued.

The first and most important result of the science of Embryology was one for which the scientific world was wholly unprepared. Down to our own century, nothing could have been farther from the conception of anatomists and physiologists than the fact now generally admitted, that all animals, without exception, arise from eggs. Though Linnæus had already expressed this great truth in the sentence so often quoted,— “ Omne vivum ex ovo,” — yet he was not himself aware of the significance of his own statement, for the existence of the Mammalian egg was not then dreamed of. Since then the discoveries of von Baer and others have shown not only that the egg is common to all living beings without exception, from the lowest Radiate to the highest Vertebrate, but that its structure is at first identical in all, composed of the same primitive elements and undergoing exactly the same process of growth up to the time when it assumes the special character peculiar to its kind. This is unquestionably one of the most comprehensive generalizations of modern times.

In common parlance, we understand by an egg something of the nature of a hen’s egg, a mass of yolk surrounded with white and inclosed in a shell. But to the naturalist, the envelopes of the egg, which vary greatly in different animals, are mere accessories, while the true egg, or, as it is called, the ovarian egg, with which the life of every living being begins, is a minute sphere, uniform in appearance throughout the Animal Kingdom, though its intimate structure is hardly to be reached even with the highest powers of the microscope. Some account of the earlier stages of growth in the egg may not be uninteresting to my readers. 1 will take the egg of the Turtle as an illustration, since that has been the subject of my own especial study; but, as I do not intend to carry my remarks beyond the period during which the history of all vertebrate eggs is the same, they may be considered of more general application.

It is well known that all organic structures, whether animal or vegetable, are composed of cells. These cells consist ot an outside bag inclosing an inner sac, and within that sac there is a dot. The outer bag is filled with semi-transparent fluid, the inner one with a perfectly transparent fluid, while the dot is dark and distinct. In the language of our science, the outer envelope is called the Ectoblast, the inner sac the Mesoblast, and the dot the Entoblast. Although they are peculiarly modified to suit the different organs, these cells never lose this peculiar structure; it may be traced even in the long drawnout cells of the flesh, which are like mere threads, but yet have their outer and inner sac and their dot, — at least while forming.

In the Turtle the ovary is made up of such cells, spherical at first, but becoming hexagonal under pressure, when they are more closely packed together. Between these ovarian cells the egg originates, and is at first a mere granule, so minute, that, when placed under a very high magnifying power, it is but just visible. This is the incipient egg, and at this stage it differs from the surrounding cells only in being somewhat darker, like a drop of oil, and opaque, instead of transparent and clear like the surrounding cells. Under the microscope it is found to he composed of two substances only : namely, oil and albumen. It increases gradually, and when it has reached a size at which it requires to be magnified one thousand times in order to be distinctly visible, the outside assumes the aspect of a membrane thicker than the interior and forming a coating around it. This is owing not to an addition from outside, but to a change in the consistency of the substance at the surface, which becomes more closely united, more compact, than the loose mass in the centre. Presently we perceive a bright, luminous, transparent spot on the upper side of the egg, near the wall or outer membrane. This is produced by a concentration of the albumen, which now separates from the oil and collects at the upper side of the egg, forming this light spot, called by naturalists the Purkinjean vesicle, after its discoverer, Purkinje. When this albuminous spot becomes somewhat larger, there arises a little dot in the centre,—the germinal dot, as it is called. And now we have a perfect cell-structure, differing from an ordinary cell only in having the inner sac, inclosing the dot, on the side, instead of in the centre. The outer membrane corresponds to the Ecloblast, or outer cell sac, the Purkinjean vesicle to the Mesoblast, or inner cell sac, while the dot in the centre answers to the Entoblast. When the Purkinjean vesicle has completed its growth, it bursts and disappears; but the mass contained in it remains in the same region, and retains the same character, though no longer inclosed as before.

At a later stage of the investigation, we see why the Purkinjean vesicle, or inner sac of the egg, is placed on the side, instead of being at the centre, as in the cell. It arises on that side along which the axis of the little Turtle is to lie,—the opposite side being that corresponding to the lower part of the body. Thus the lighter, more delicate part of the substance of the egg is collected where the upper cavity of the animal, inclosing the nervous system and brain, is to be, while the heavy oily part remains beneath, where the lower cavity, inclosing all the organs of mere material animal existence, is afterwards developed. In other words, when the egg is a mere mass of oil and albumen, not indicating as yet in any way the character of the future animal, and discernible only by the microscope, the distinction is indicated between the brains and the senses, between the organs of instinct and sensation and those of mere animal functions. At that stage of its existence, however, when the egg consists of an outer sac, an inner sac, and a dot, its resemblance to a cell is unmistakable ; and, in fact, an egg, when forming, is nothing but a single cell. This comparison is important, because there are both animals and plants which, during their whole existence, consist of a single organic cell, while others are made up of countless millions of such cells. Between these two extremes we have all degrees, from the innumerable cells that build up the body of the highest Vertebrate to the single-celled Worm, and from the myriad cells of the Oak to the single-celled Alga.

But while we recognize the identity of cell-structure and egg-structure at this point in the history of the egg, we must not forget the great distinction between them, — namely, that, while the cells remain component parts of the whole body, the egg separates itself and assumes a distinct individual existence. Even now, while still microscopically small, its individuality begins ; other substances collect around it, are absorbed into it, nourish it, serve it. Every being is a centre about which many other things cluster and converge, and which has the power to assimilate to itself the necessary elements of its life. Every egg is already such a centre, differing from the cells that surround it by no material elements, but by the principle of life in which its individuality consists, which is to make it a new being, instead of a fellow-cell with those that build up the body of the parent animal and remain component parts of it. This intangible something is the subtile element that eludes our closest analysis; it is the germ of the immaterial principle according to which the new being is to develop. The physical germ we see ; the spiritual germ we cannot see, though we may trace its action on the material elements through which it is expressed.

The first change in the yolk, after the formation of the Purkinjean vesicle, is the appearance of minute dots near the wall at the side opposite the vesicle. These increase in number and size, but remain always on that half of the yolk, leaving the other half of the globe clear. One can hardly conceive the beauty of the egg as seen through the microscope at this period of its growth, when the whole yolk is divided, with the dark granules on one side, while the other side, where the transparent halo of the vesicle is seen, is brilliant with light. With the growth of the egg these granules enlarge, become more distinct, and under the microscope some of them appear to be hollow. They are not round in form, but rather irregular, and under the effect of light they are exceedingly brilliant. Presently, instead of being scattered equally over the space they occupy, they form clusters, — constellations, as it were, — and between these clusters are clear spaces, produced by the separation of the albumen from the oil.

At this period of its growth there is a wonderful resemblance between the appearance of the egg, as seen under the microscope, and the firmament with the celestial bodies. The little clusters or constellations are unequally divided: here and there they are two and two like double stars, or sometimes in threes or fives, or in sevens, recalling the Pleiades, and the clear albuminous tracks between are like the empty spaces separating the stars. This is no fanciful simile: it is simply true that such is the actual appearance of the yolk at this time ; and the idea cannot but suggest itself to the mind, that the thoughts which have been at work in the universe are collected and repeated here within this little egg, which ofters us a miniature diagram of the firmament. This is one of the first changes of the yolk, ending by forming regular clusters with a sort of network of albumen between, and then this phase of the growth is complete.

Now the clusters of the yolk separate, and next the albumen in its turn concentrates into clusters, and the dark bodies, which have been till now the striking points, give way to the lighter spheres of albumen between which the clusters are scattered. Presently the whole becomes redissolved : these stages of the growth being completed, this little system of worlds is melted, as it were : but while it undergoes this process, the albuminous spheres, after being dissolved, arrange themselves in concentric rings, alternating with rings of granules, around the Purkinjean vesicle. At this time we are again reminded of Saturn and its rings, which seems to have its counterpart here. These rings disappear, and now once more out of the yolk mass loom up little dots as minute as belbre; hut they are round instead of angular, and those nearest the Purkinjean vesicle are smaller and clearer, containing less of oil than the larger and darker ones on the opposite side. From this time the yolk begins to take its color, the oily cells assuming a yellow tint, while the albuminous cells near the vesicle become whiter.

Up to this period the processes in the different cells seem to have been controlled by the different character of the substance of each; but now it would seem that the changes become more independent of physical or material influences, for each kind of cell undergoes the same process. They all assume the ordinary cell character, with outer and inner sac, — the inner sac forming on the side, like the Purkinjean vesicle itself; but it does not retain this position, for, as soon as its wall is formed and it becomes a distinct body, it floats away from the side and takes its place in the centre. Next there arise within it a number of little bodies crystalline in form, and which actually are wax or oil crystals. They increase with great rapidity, the inner sac or mesoblast becoming sometimes so crowded with them that its shape is affected by the protrusion of their angles. This process goes on till all the cells are so filled by the mesoblast, with its myriad brood of cells, that the outer sac or ectoblast becomes a mere halo around it. Then every mesoblast contracts; the contraction deepens, till it is divided across in both directions, separating thus into four parts, then into eight, then into sixteen, and so on, till every cell is crowded with hundreds of minute mesoblasts, each containing the indication of a central dot or entoblast. At this period every yolk cell is itself like a whole yolk ; for each cell is as full of lesser cells as the yolk-bag itself.

When the mesoblast has become thus infinitely subdivided into hundreds of minute spheres, the ectoblast bursts, and the new generations of cells thus set free collect in that part of the egg where the embryonic disk is to arise. This process of segmentation continues to go on downward till the whole yolk is taken in. These myriad cells are in fact the component parts of the little Turtle that is to be. They will undergo certain modifications, to become flesh-cells, blood-cells, brain-cells, and so on, adapting themselves to the different organs they are to build up ; but they have as much their definite and appointed share in the formation of the body now as at any later stage of its existence.

We are so accustomed to see life maintained through a variety of complicated organs that we are apt to think this the only way in which it can be manifested ; and considering how closely life and the organs through which it is expressed are united, it is natural that we should believe them inseparably connected. But embryological investigations have shown us that in the commencement none of these organs are formed, and yet that the principle of life is active, and that even after they exist, they cannot act, inclosed as they are. In the little Chicken, for instance, before it is hatched, the lungs cannot breathe, for they are surrounded by fluid, the senses are inactive, for they receive no impressions from without, and all those functions establishing its relations with the external world lie dormant, for as yet they are not needed. But they are there, though, as we have seen in the Turtle’s egg, they were not there at the beginning. How, then, are they formed ? We may answer, that the first function of every organ is to make itself. The building material is, as it were, provided by the process which divides the yolk into innumerable cells, and by the gradual assimilation and modification of this material the organs arise. Before the lungs breathe, they make themselves ; before the stomach digests, it makes itself; before the organs of the senses act, they make themselves ; before the brain thinks, it makes itself; in a word, before the whole system works, it makes itself; its first office is self-structure.

At the period described above, however, when the new generations of cells are just set free and have taken their place in the region where the new being is to develop, nothing is to be seen of the animal whose life is beginning there, except the filmy disk lying on the surface of the yolk. Next come the layers of white or albumen around the egg, and last the shell which is formed from the lime in the albumen. There is always more or less of lime in albumen, and the hardening of the last layer of white into shell is owing only to the greater proportion of lime in its substance. In the layer next to the shell there is enough of lime to consolidate it slightly, and it forms a membrane ; but the white, the membrane, and the shell have all the same quality, except that the proportion of lime is more or less in the different layers.

But, as I have said, the various envelopes of eggs, the presence or absence of a shell, and the absolute size of the egg, are accessory features, belonging not to the egg as egg, but to the special kind of being from which the egg has arisen and into which it is to develop. What is common to all eggs and essential to them all is that which corresponds to the yolk in the bird’s egg. But their later mode of development, the degree of perfection acquired by the egg and germ before being laid, the term required for the germ to come to maturity, as well as the frequency and regularity of the broods, are all features varying with tiie different kinds of animals. There are those that lay eggs once a year at a particular season and then die; so that their existence may be compared to that of annual plants, undergoing their natural growth in a season, to exist during the remainder of the year only in the form of an egg or seed. The majority of Insects belong to this category, as do also our large Jelly-Fishes; many others have a slow growth, extending over several years, during which they reach their maturity, and for a longer or shorter time produce broods at fixed intervals; while others, again, reach their mature state very rapidly, and produce a number of successive generations in a comparatively short time, it may be in a single season.

I do not intend to enter upon the chapter of special differences of development among animals, for in this article I have aimed only to show that the egg lives, that it is itself the young animal, and that the vital principle is active in it from the earliest period of its existence. But I would say to all young students of Embryology that their next aim should be to study those intermediate phases in the life of a young animal, when, having already acquired independent existence, it has not yet reached the condition of the adult. Here lies an inexhaustible mine of valuable information unappropriated, from which, as my limited experience has already taught me, may be gathered the evidence for the solution of the most perplexing problems of our science. Here we shall find the true tests by which to determine the various kinds and different degrees of affinity which animals now living bear not only to one another, but also to those that have preceded them in past times. Here we shall find, not a material connection by which blind laws of matter have evolved the whole creation out of a single germ, but the clue to that intellectual conception which spans the whole scries of the geological ages and is perfectly consistent in all its parts. In this sense the present will indeed explain the past, and the young naturalist is happy who enters upon his life of investigation now, when the problems that were dark to all his predecessors have received new light from the sciences of Palæontology and Embryology.