500 



THE THEORY OF EVOLUTION 



most reasonably interpreted by evolution. In vertebrates, there 

 are two pairs of limbs, a body, head, tail, and various internal 

 organs, always laid down according to a similar general plan, but 

 with endless modifications in relation to the mode of life. It is, 

 therefore, possible to construct a generalized plan of a vertebrate 

 (c/. Fig. 29, p. 50), representing not any particular vertebrate but 

 vertebrates in general. The same may be done with other groups, 

 as in the accounts of the general body plan in the several phyla, 

 given in preceding chapters. Thus, the ccelenterate plan, the 

 annulate plan, the arthropod plan, and the vertebrate plan may be 



constructed. More specific 

 resemblances are seen in the 

 corresponding parts of the 

 body when comparisons are 

 made among the animals of a 

 single phylum. The fhpper of 

 a whale, the wing of a bird or 

 a bat, the fore limb of a horse, 

 and the arm of a man all 

 show the same plan of struc- 

 ture despite differences (cf. 

 Figs. 262 to 264). Similar 

 resemblances between parts 

 could be cited from every 

 group of animals. This fact 

 of resemblancein plan, whether 

 of the body as a whole or of 

 its special parts, is the great 

 principle of homology, which appears in every phylum of the 

 Animal Kingdom. It may even be extended to comparisons 

 between the several phyla (c/. Fig. 117, p. 240). Such are the 

 facts that any one can see for himself by the dissection and com- 

 parative study of existing animals, just as one might study the 

 comparative anatomy of chairs or houses or automobiles. If there 

 is any point in dispute one can look again and determine the 

 exact relationships. 



For the purposes of this discussion, the question is: What do 

 these obvious facts indicate regarding the history of animal fife? 

 The preevolutionary explanation was that each species of animal, 

 though created separately, was nevertheless formed in accordance 



Fig. 263. — Flipper of a whale com- 

 pared with hand of man (cf. Fig. 18, 

 p. 39). 



(From Romanes, " Darwin and after 

 Darwin," copyright, 1S96, by Open Court 

 Publishing Co., reprinted by permission.) 



