STRUCTURE OF THE VERTEBRATES 7 



proceed in different ways and at different rates in isolated races 

 of the group. 



4. Adaptive Radiations refers to the specializations which 

 adapt different divisions of a group to various environments. 

 The whale and monkey are both mammals though adapted to 

 entirely different conditions. This tendency of an ancestral stock 

 to radiate into many evolutionary lines is frequently called 

 divergence. Any group which gives rise to another is ancestral 

 to those which follow. However, the process of divergence can- 

 not be carried on indefinitely for races, like individuals, die. 

 They may become so specialized that they cannot survive the 

 slightest change of environment, and it is therefore apparent 

 that the possibilities of divergence are greatest in a general- 

 ized race. 



5. Degenerate conditions are merely a phase of specialization. 

 It implies a loss of functions which once were present. If an 

 animal during its embryonic life has a well developed nervous 

 system, and then during the course of its development loses much 

 of the function of its coordinating structures, we think of it as 

 a degenerate adult. In racial history the same applies. A Boa 

 constrictor, with only diminutive structures in place of hind legs, 

 is a specialized reptile with degenerate legs. 



6. Homology. The above necessitates a consideration of homol- 

 ogy, for before one can say that a snake has degenerate legs 

 it must be admitted that his racial ancestors had functional legs. 

 The proof of this lies in the fact that the tiny appendages of a 

 Boa are developmentally similar to those of a lizard, and the 

 fossil history confirms the embryological evidence. As a result 

 the evolutionist thinks of them as homologous structures. 

 Homology does not necessarily imply any functional relation- 

 ship, but a developmental similarity; that is, two structures are 

 homologous if they evolved from the same organ in a racial 

 ancestor. In this way the wing of a bird and the hand of man 

 are homologous, though functionally different. Their racial and 

 embryological history show that they arose from a similar primi- 

 tive structure. 



7. Analogy. Analogous structures are those which serve a 

 similar function, for example the wing of a bird and the wing of 

 a butterfly. They are structurally, racially and embryological ly 



