Page:Popular Science Monthly Volume 37.djvu/525

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EVOLUTION AND DISTRIBUTION OF ANIMALS.

of the parent species. Again, colony after colony of species after species may be destroyed by other species or by uncongenial surroundings.

Only in the most general way can the history of any species be traced; but, could we know it all, it would be as long and as eventful a story as the history of the colonization and settlement of North America by immigrants from Europe. Each region where animals or plants can live has been thousands of times discovered, its colonization a thousand times attempted. In these efforts there is no co-operation. Every individual is for himself, every struggle a struggle of life and death; to each species each member of every other species is an alien and ah enemy.

The arctic birches serve as one illustration only of the spread and change of organisms in the face of a barrier apparently insurmountable. I can not enter into detail as to the many ways in which individuals manage to cross the barriers which usually limit the species. These ways are as varied as the creatures themselves, and infinitely more varied than the barriers. It is enough to say that organisms have extended their range in regions where their existence is possible. Here, by the long-continued process of adjustment to circumstances, with the incessant destruction of the unadapted, these organisms have become so well fitted to their surroundings as to give rise to the popular impression that each species now inhabits that part of the world best fitted for its occupation. Yet the very reverse of this must be true, for in the growth of any species it is these features of adaptation which are the last to appear. If, as anatomists now teach, the history of the individual is an epitome of the history of the group to which the individual belongs, then adaptive characters appearing late in the growth of the individual must have appeared late in the history of the group. They are the last changes made in the organism—mere after-thoughts in the work of creation.

For example, the long pectoral fins of the flying-fish enable it to make great leaps through the air, after the manner of the grasshopper. Yet we can not say that the flying-fish was meant to be the bird among fishes, for its nearest relatives are without wings, and the wing-development is one of the last acquisitions of the individual. Its flight is simply an exaggeration of the leaping or skimming which related forms with shorter fins accomplish. The growth of the fins goes on with the increase of this power, and greater power comes with the growth of the fins.

To my mind the strongest arguments for the theory of development are those drawn from the changing character of the species themselves.

No phase in the history of systematic science is more instruct-