210 EVOLUTION AND ANIMAL LIFE 



phylogenetic characters, the two forms of trout are one species. 

 We have reason to believe that fry from the bay would grow up 

 as dwarfs in the brook, and that the fry from the brook would be 

 gray giants if developed in the sea. 



But it is also supposable that in the complete isolation of the 

 brook fishes, with free interbreeding, there would be some sort 

 of phylogenetic bond. There may be a genuine subspecies, 

 tudes, characterized not by small size, slender form, and bright 

 colors, but by other traits, which no one has found because no 

 one has looked deeply enough. 



In no group of vertebrates are the life characters more plastic 

 than among the trout. The birds have traits far more definitely 

 fixed. Yet differences in external conditions must produce cer- 

 tain results. We should not venture to suggest that the dusky 

 woodpeckers or chickadees of the rainy forests of the northeast 

 and northwest are purely ontogenetic species or that they should 

 be erased from the systematic lists. But it will be a great ad- 

 vance in ornithology when we know what they really are and 

 when we understand the real nature of the small-bodied, large- 

 billed, southern races of other species of birds. It would be 

 worth while to know if these are really ontogenetic purely, or 

 if they are phylogenetic through "progressive heredity," the 

 inheritance of acquired characters, such as are produced by 

 the direct effects of climate or as the reaction from climatic 

 influences. Or again may there be a real phylogenetic bond 

 through geographical segregation, its evidences obscured by 

 the more conspicuous traits induced by like experiences? Or 

 are there other influences still more subtle involved in the 

 formation of isohumic or isothermic subspecies? 



To sum up, there is no convincing evidence that the direct 

 influence of environment is a factor in the separation of species, 

 except as its results may be acted upon by natural selection. 

 We have no proof to show that the environment of one genera- 

 tion determines the heredity of the next and yet perhaps 

 most naturalists feel that the effects of extrinsic influences work 

 their way into the species, although a mechanism by which 

 this might be accomplished is as yet unknown to us. 



