EXTENSION OF EVOLUTIONARY THEORIES 34 1 



Lamarckian principle, and as we have rejected the one, 

 we need not linger in discussing the other. However, 

 we will say a few words about orthogenesis, as in 

 speaking of the Lamarckian principle we dealt almost 

 exclusively with the inheritance of changes brought 

 about by the actions of the animals, their energy and 

 efforts. 



Animals are said to be permanently affected chiefly 

 by the following external agencies : climate, the char- 

 acter of the soil, and food. Thus the stimulus of cold 

 may provoke a thicker coat of hair in many animals. 

 This will be transmitted to the young, and as the cold 

 continues to affect them in turn the hair will grow longer 

 and longer. In this way the animals will vary in one 

 definite direction as long as the stimulus lasts. 



It has been alleged in support of this view that many 

 poisons make organisms more sensitive the longer they 

 act on them. That, however, only shows an increase 

 of the action of a stimulus during the individual life, not 

 that the increase is inherited. 



But there is an experiment that seems to prove the 

 inheritance of a stimulated effect. In this the pupae of 

 a butterfly, the large tortoise-shell, were exposed to a 

 cold of 15° Fahr. The butterflies that were developed 

 from them were much darker than the normal specimens. 

 A couple of these modified animals were crossed, and 

 from the ova, which were developed under entirely 

 natural conditions, came caterpillars that afterwards 

 changed into tortoise-shells of a shade similar to that 

 of the parents, or slightly lighter. 



Had the modification ot the parents by cold been 



