EVOLUTION AND HEREDITY 347 



survival of structures which at one time in racial history 

 were functional. The principle of "homology" has also 

 been of value hi supporting evolutionary theories. For 

 example, all vertebrates have the same series of bones 

 arranged in the same way in their limbs, but there is great 

 variation in proportions. It is difficult to explain why 

 such diverse organs as a frog's leg, a whale's flipper, a 

 bird's wing, and a horse's foreleg should show homology, 

 unless it is assumed that all vertebrates came from a com- 

 mon ancestral stock. 



Physiology and Ecology. There are many facts in 

 physiology and in the relationships of animals that are 

 readily interpreted by evolutionary theory. The functional 

 activity of the nervous systems shows steady progress 

 from the simpler to more complex animals. Starting in 

 sponges with separate and independent effectors, simple 

 reflexes are established through receptors and effector cells, 

 there is combination and coordination of such reflexes by 

 adjusters, and finally the more or less complete domination 

 of primitive effectors and receptors by a central nervous 

 mechanism. 



It is hard to believe that such intricate relations as exist 

 between parasite and host have been thus from the begin- 

 ning of things. If, however, we assume that a liver fluke 

 (Fig. 74) was first a free-living aquatic worm somewhat like 

 a planarian (Fig. 73), that it gradually took up a parasitic 

 life during succeeding generations and developed the 

 ability to live and multiply during larval stages in the 

 abundant nourishment provided by a snail's body, the 

 case is much more understandable. 



Experimental Evolution. To any thoughtful person the 

 proof of evolution would seem to lie in experiments and 

 many attempts have been made to secure positive evidence 

 in this way, but until the past few years there has been 

 little that is satisfying. Recent investigators have con- 

 tributed some valuable facts as to how animals vary and 

 how particular hereditary characters are transmitted from 



