3 1 8 Problems of Organic Adaptation 



The migratory habits of animals are no less wonderful. 

 At the breeding season myriads of shad and salmon migrate 

 from the sea far toward the sources of fresh-water streams 

 where the young may grow up in comparative safety, and 

 although very few of the adult animals ever get back to the 

 sea, yet this same instinct for migration possesses every new 

 generation as it did the old one. The immemorial migra- 

 tions of certain birds, going north in spring and south in 

 autumn, are equally wonderful. The value of such an instinct 

 to the birds is easily understood; but how did it arise, what 

 series of natural causes can explain such an instinct? These 

 adaptive instincts are no exceptions but only striking illus- 

 trations of a universal phenomenon among organisms. How 

 can such useful and apparently intelligent and purposive 

 adaptations be explained? Are intelligence and purpose in 

 man fundamentally different from this adaptive behavior 

 of animals ? Apparently many gradations exist between 

 these two, and in the development of the human individual 

 every intermediate step is found between mere tropisms 

 at one extreme and intelligence at the other. If tropisms 

 and instincts are generally adaptive, are not intelligence and 

 purpose higher and more complicated forms of adaptation? 



6. Cellular Adaptations 



Adaptations are found not only in gross structures and 

 functions but also in the most minute, not only in tissues 

 and cells but also in the smallest parts of cells. For example, 

 what is there in the whole world more remarkable than the 

 complex mechanism of nuclear division? We now know that 

 the material basis of heredity is located in certain portions 

 of the nucleus, the chromosomes, and, if this material is 

 to be equally distributed in development to all portions of 

 the body, each chromosome must be divided with exact 



