THEORIES OF EVOLUTION 139 



brief, that mechanical stresses exert a stimulating 

 influence on the development of the skeletal parts 

 involved. The repeated impact of the feet of 

 cursorial animals on the ground, for example, is 

 supposed to have led to the lengthening of the 

 bones of the limbs. Bones have been shown to 

 increase in size in response to such influences, but 

 in other cases pressure has resulted in the local 

 limitation of bone development, hence the theory 

 seems to be a valid explanation of change in the 

 skeleton as a result of mechanical stresses. It is, 

 of course, subject to the same fundamental limita- 

 tion as other Lamarckian theories in that it does 

 not explain the cumulative action of the process 

 through successive generations. 



The more prominent hypotheses for the associa- 

 tion of bodily responses with the hereditary mech- 

 anism are Darwin's pangenesis, ^^ Semon's mneme 

 theory, ^^ and Rignano's centro-epigenesis.^^ There 

 is little use in a detailed criticism of these views. 

 Their chief and common limitation is their use 

 of elaborate and purely hypothetical mechanisms. 

 Pangenesis is of only historical interest; the other 

 two hypotheses are logical, but like all expressed 

 views of this subject, they yield readily to the 



" Animals and Plants under Domestication, Second Edition, Vol. II, 

 Ch. XXVII, 1887. 



1^ Die Mneme als erhaltendes Prinzip im Wechsel des Organischen Geschehens, 

 1904. 



^^ TJher die Vererbung erworbener Eigenschajten, Hypothese einer Zentro- 

 epigenese, 1907. 



