It was long ago noticed—first, I believe, by Louis Agassiz—that the life on the different continents exhibited different rates of advance in gradation of structure. He called attention to the fact that the existing forests of North America were essentially like the fossil forests of Switzerland, which flourished during the middle tertiaries; that the mammalian life of South America found many representatives among the earlier tertiaries of Europe; and that the. existing flora and fauna of Australia can best be compared with the Jurassic life of the European seas and shores. This last-named feature has been frequently dwelt upon, and is one of the most striking facts in the distribution of organic life. Carrying farther the observations made by Agassiz, I have endeavored to attain a more precise result by taking the organic life of Europe as a standard, and then seeking in each continent the forms which had been represented in the past life of Europe, but which had been over-ridden in the rapid on-going of the organic life of that continent. Placing against the name of each continent the forms still existing there which could be regarded as obsolete European forms, I found that the series so obtained showed North America to be next to Europe in the advance of its organic life, Asia and Africa next, South America below these, and Australia the lowest in the series. That is to say, Europe has fewest ancient types; North America has a rather fuller share of antique forms; Asia and Africa more than the North Atlantic continents; South America is still more archaic in its life; and Australia is thicker peopled with archaic organic forms than any of the other before-mentioned areas. If now we could find that there was a corresponding series in the variety of physical conditions in these several continents, we should have an important confirmation of the hypothesis. Here again we must have recourse to indirect methods. It is not possible to measure with accuracy the variations of environment on the surface of the several continents. Generations, possibly centuries, will pass away before these conditions are known well enough for detailed comparison. An observation of Ritter, however, makes it possible for us to attain our end: he noticed that the extent of shore line compared with the square-mile area of the several continents varied greatly, — Europe having far more shore line than any other continent. If we desire to institute this comparison between shore line and internal area of the continents complete, we must reduce them to the same area, preserving their form, and then compare their shore lines with their internal areas. This I have approximately done, and find that the succession of continents in this series is essentially the same as in the series given us by the number of ancient forms retained on the several areas, — Europe coming first, North America next, Asia and Africa next, and near each other, then South America, and last Australia. It will be evident to the reader that the ratio between the length of shore line and the internal surface will be a fair measure of the variety of that surface. Nearly every mountain chain in Europe contributes to the diversity of its outline; the sea serving to give one plane of comparison by which we may measure the variety of configuration of the several continents. We may reasonably suppose that the various mountain chains in the other continents are as fairly indicated by the accidents of the shore line. It would be better if we could have all the contour lines of all the continents, but there is every reason to believe that the one given by the sea in its present position fairly represents the average diversity of surface conditions. The general fact may therefore be accepted that the continents have their rate of advance in the organic progress reasonably well measured by the variety of their surface conditions.
This brings us to consider another element in the conditions of the continent, namely, new changes of climate. Out of the many alterations which the climate of the world has undergone, it is that set of changes alone which affect the general aspect that writes a record which is as yet intelligible to us. The effect of these variations on the organic life of the land is demonstrably great, and the regions subjected to them must be expected to exhibit many traces of their results in the condition of the life they bear. Only Europe and North America have taken the full brunt of the last glacial periods, and it is here that we find the most important modifications in organic life. It is far from our purpose to do more than touch upon these great questions; still it is not without value that we see that all these forces, which we know to be effective in producing great diversities of the conditions of organic life, have operated with the most power upon those continents which are the farthest in their advance towards the highest level of life, and that the continent which has had the least of these diversifying accidents remains singularly backward in all its types of life.