EVOLUTION AND DISTRIBUTION OF ANIMALS. 509 



of the parent species. Again, colony after colony of species after 

 species may be destroyed by other species or by uncongenial sur- 

 roundings. 



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 creat- 

 ures 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-con- 

 tinued 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 charac- 

 ters 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 grass- 

 hopper. 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 develop- 

 ment are those drawn from the changing character of the species 

 themselves. 



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



