ORGANIC EVOLUTION 287 



Direct adapta tion seems especially to explain such classes 

 of facts as are furnished by geographic distribution, especially 

 of island life, by parallelisms, by mimicry, by degeneration, 

 etc. Let us illustrate by means of the parallelism of the 

 swift and the swallow. How have these birds that are so 

 different structurally, become so very much alike in form, in 

 flight, and in foraging habits that it requires something of 

 an ornithologist to distinguish between them? It is a 

 peculiar field for bird life that they occupy. Above the 

 ponds and lakes there hovers a teeming population of 

 midges and other little insects excellent for food. How 

 have these two, of all the groups of birds, become so finely and 

 so similarly fitted to profit by it? Is it more likely that 

 internal forces automatically produced such external like- 

 ness built upon persistent structural unlikeness, or that a 

 common environment, imposing common conditions, has, 

 acting through long ages, shaped to common form and 

 function those parts with which it came most directly in 

 contact? When we note the numerous details of similarity 

 that are coupled with convincing evidence of diverse origin, 

 we incline to doubt that these likenesses can be wholly due 

 to internal spontaneous developmental tendencies, just as 

 we doubt the originality of two essays that show many 

 points of correspondence. Surely internal forces would 

 modify internal form, as well as external. The impress of 

 environment appears in this that it is the outside of organ- 

 isms that show all the special fitnesses to the environing 

 conditions. As a distinguished American zoologist has 

 graphically stated it, "The inside of an animal shows what 

 it is; the outside shows where it has been." 



Environmental influence comes out most conspicuously 

 where different environments impose very different condi- 

 tions; as, for example, when part of a group passes over 

 from terrestrial to aerial or to aquatic life. Some such cases 

 will be taken up for special study in Chapter VI. 



