Evolution 323 



The Origin of Species is undoubtedly one of the most important 

 pieces of work ever published. Not only did it influence biological think- 

 ing, but the concepts have spread into all fields of thought. The main 

 points of Darwin's thesis were that animals tended to reproduce and 

 outrun their food supplies; thus many must perish. Among animals, 

 variations continuously occurred. Of these variations, only those 

 forms possessing the favorable ones could survive. "Survival of the 

 fittest" thus accounted for the continuation of new forms. Boldest of 

 all, Darwin finally fit man into the animal kingdom and showed that 

 he, too, had evolved. All of this brought forth a storm of criticism 

 which has never been equalled. Only recently has it died down. 



THE EVIDENCES OF EVOLUTION 



The whole organic world gives evidence of the fact that evolution 

 has occurred. While all phases of zoology demonstrate this, some 

 do it more clearly than do others. 



Comparative Anatomy. — The study of the comparative anatomy 

 of the vertebrates gives evidence of the evolution of the various 

 parts. In particular, the forelimbs with their many modifications may 

 be cited. In all of the classes of vertebrates, these limbs have a common 

 basic structure, that is. the parts are homologous. However, they are 

 often greatly modified in the adult. The forelimbs of the fish become 

 adapted for swimming; those of the birds and bats for flying; of man, 

 for grasping; and of other mammals for walking, running, digging, or 

 climbing. Despite this seeming disparity of use and appearance, these 

 limbs show that there was common ancestry for all the vertebrates. 



In examining animals for examples of homologies, care must be 

 taken not to confuse analogy with homology. Analogous structures are 

 structures with a common function but with different origin and struc- 

 ture (Fig. 33). Thus the wings of birds and bats are homologous; 

 those of the bird and insect are analogous. 



Many other structures of vertebrates also give excellent evidence 

 of common origin. Among these are the aortic arches, the structure of 

 the heart (Fig. 47), the general arrangement of organs, the structure 

 of the brain (Fig. 59), and the teeth. Even among the invertebrates 

 many homologous structures are observable. By use of these like or 

 similar structures, it is possible to learn the relationships of the many 

 animal phyla. 



