4o8 ELEMENTS OF BIOLOGY 



pose that the evolutionary process has been halted. Furthermore^ 

 the fact that so many groups have become extinct and that as the 

 centuries of the present age go by some modern animals have be- 

 come extinct, shows clearly that we are not justified in assuming 

 that any modern animals, including Man, are exempt from the pos- 

 sibility of extinction. 



Evidence from Comparative Anatomy. Of the wide va- 

 riety of facts of adult anatomy that can be interpreted only on the 

 basis of an evolutionary series, only a few are listed here. 



The basic body plan of many lower animals, which consists 

 essentially of a tube-within-a-tube arrangement, as in Nemathel- 

 minthes, Annelida, and Arthropoda, is also the basic plan of higher 

 animals, including Man. 



Metamerism, or segmentation, is an architectural feature that is 

 common to several phyla, most sharply defined in the adults of the 

 lower forms and in the embryos of the higher forms. The various 

 types of animals which exhibit this basic character also exhibit wide 

 divergencies and alterations that can be explained rationally only 

 on the interpretation that these divergencies and alterations in 

 themselves represent the evolution of the metameric plan within the 

 particular groups. 



Unity of Plan within a Group: For example, all members of 

 the class Insecta contain the same number of segments; moreover, 

 all members of the class show three body divisions, head, thorax, 

 and abdomen. All members of the sub-phylum Vertebrata (except 

 cyclostomes) have four appendages and only four. And so on; the 

 list might be extended almost indefinitely. 



Vestigial and Rudimentary Structures: The original digits of 

 the ancestor of the horse, Eohippus, are found as the splint bones 

 of the modern horse. The human vermiform appendix is a vestige 

 of the end of the long caecum found in other mammals lower in 

 the scale. The muscles of the human scalp and of the human ear 

 are present in a reduced form and are usually not within the con- 



