202 EVOLUTION AND ANIMAL LIFE 



very slight as compared with the cumulative effects of selection 

 in breeding, and it is safe to assert that there is no such whole- 

 sale and immediate modification of the germinal plasm due to 

 nutrition as some people seem to suppose." 



As a matter of fact experiment has shown that the results of 

 dwarfing due to lack of food are shown for three generations in 

 silkworms (these subsequent broods, of larvae being full fed 

 but producing dwarfed moths). But with succeeding genera- 

 tions the moths became larger and resumed their normal ap- 

 pearance. 



Mutilations of any sort are not inherited. The tails of 

 sheep have been cut off for countless generations. Yet each 

 lamb is born with a tail. This law holds good for docked tails, 

 docked ears, pierced ears, and the many mutilations to which 

 domestic animals and men have been subject since the begin- 

 ning of civilization. 



Influences of climate, of heat, of cold are not inherited so 

 far as experiment shows, nor has it been made clear that any 

 extrinsic influence exerted on the individual really modifies 

 the forces of heredity. Even Lamarck admits this. He ob- 

 serves: "Circumstances change the forms of animals. But I 

 must not be taken literally, for environment can effect no direct 

 changes whatever upon the organization of animals." 



In Spencer's view, the phenomena of instinct are to be ex- 

 plained as the inheritance of habits of the individual. The 

 Neo-Darwmians see in the adaptations of instinct only the re- 

 sults of natural selection acting upon the endless variations to 

 which individual instincts are subject. In most cases the latter 

 view seems the most probable. In some cases it hardly offers 

 a plausible explanation. 



The young mocking bird shows an inborn dread of owls and 

 cats, while it is relatively indifferent to the presence of dogs or 

 chickens. It seems hardly reasonable to suppose that all 

 mocking birds without this instinct of dread for these particular 

 animals have been destroyed, while the others have survived. 

 Still more deep seated is the dread of snakes possessed by all the 

 monkey species known to us, as well as by their human allies. 

 Most men and most monkeys have a different feeling in regard 

 to snakes from that exhibited toward any other sort of animals. 

 This feeling is inborn. It may be suppressed, but not often 

 wholly conquered. To call it an inherited experience is easy, 



