542 



THE CAUSES OF EVOLUTION 



of use and disuse and of environment are actually inherited. If 

 such inheritance does not occur, there is no real evolution, since 

 each generation starts as did the parents (c/. Fig. 299). Herein 

 lies the weakness of the entire scheme. No one has yet presented 

 evidence that is generally accepted, for the inheritance of such 

 acquired characteristics. It would seem possible to obtain such 

 evidence if the theory holds. Lamarckians no longer believe that 

 the disuse following mutilation causes hereditary degeneration. 

 As one writer puts it, " wooden legs do not run in families, although 

 wooden heads may." Experiments in the destruction of parts. 



Fig. 298. — Modification of neck in giraffes. 



Left, the familiar long-necked species that lives upon the savannas and browses upon 

 the higher branches of trees; center, the okapi or forest giraffe, a species that feeds upon 

 lower branches; right, head and neck of long-necked giraffe for comparison with okapi. 

 (Photos, by courtesy of the American Museum of Natural History.) 



such as the amputation of tails in mice during many generations, 

 as well as the special functional stimulation of parts, and in the 

 effects of changed environment have given results that are con- 

 sidered negative by most investigators. The organism may 

 develop new characteristics, but when it is returned to the original 

 environment these effects are not lasting. Hence, the results of 

 actual experimentation seem to indicate that the Lamarckian 

 Theory is unsatisfactory as an explanation of the cause of evolu- 

 tion, because the characteristics thus acquired by individuals are 

 not inherited. 



A theoretical objection that arises is the mechanism of such 

 inheritance, if it be assumed to occur. An individual develops not 

 irom its parent's body, but from germ cells (cf. Fig. Ill, p. 222), 



