240 Heredity and Environment 



eyes or teeth or legs lead to the absence or weakened development 

 of these organs in future generations, seeing that inheritance must 

 be through germ cells which possess none of these structures ? 



Lack of Evidence for Inheritance of Acquired Characters. 

 But, apart from these general objections to the doctrine of the in- 

 heritance of acquired characters, there are many special difficul- 

 ties. There is little or no conclusive and satisfactory evidence in 

 favor of such inheritance. Almost all the evidence adduced serves 

 to show only that characters are acquired, not that they are 

 inherited. 



It is a matter of common observation that mutilations are not 

 inherited; wooden legs do not run in families, although wooden 

 heads do. The evidence for the inheritance of peculiarities due 

 to use or disuse is wholly inconclusive; for example, did the 

 giraffe get his long neck because he browsed on trees, or does he 

 browse on trees because he has by inheritance a long neck? Did 

 attempts to fly lead to the development of wings in birds, or do 

 birds fly because heredity has given them wings? Did life in 

 caves make cave animals blind, or did blind animals resort to 

 caves because the struggle for existence there was less severe 

 for them? The evidence is in favor of the second of each of 

 these alternatives rather than of the first. 



There still remains the question of the inheritance of certain 

 characters due to environment, though here also the most clear- 

 cut evidence is against this proposition. That unusual conditions 

 of food, temperature, moisture, etc., may affect the germ cells so 

 as to produce general and indefinite variations in offspring is 

 probable, but this is a very different thing from the inheritance 

 of acquired characters. The germ cells being a part of the paren- 

 tal organism may be modified by such changes in the environment 

 as affect the body as a whole, they may be well nourished or 

 starved, they may be modified by changed conditions of gravity, 

 salinity, pressure, temperature, etc., and these modifications of the 

 germ cells probably lead to certain general modifications of the 



