Heredity and Natural Selection. 295 



with the shortest necks would suffer most, from inabil- 

 ity to reach their food, but he says that this would not 

 affect the cells of their necks especially, but would result 

 in general disadvantage to the whole body. 



The validity of this objection cannot be denied, and I 

 do not think it would be difficult to find many instances 

 which are much more striking than the two which have 

 been referred to. 



It is very difficult to understand how our explanation 

 of the origin of vari^ition can apply to instances of modi- 

 fication in animals which, like worker bees, do not pro- 

 duce descendants. 



It is i^roper to point out, however, that these cases are 

 no more difficult to explain after our theory of heredity 

 is accepted than without it. Its acceptance does, in 

 many cases, greatly simplify our conception of natural 

 selection, and the fact that it still leaves difficulties un- 

 explained, is no reason for rejecting it, provided it does 

 not add to these difficulties. 



In the case of the giraffe it is not difficult to under- 

 stand that if circumstances should compel this animal 

 to stretch frequently after foliage almost beyond its 

 reach, this might cause hardship in the cells of the 

 neck, and thus result in the production of gemmules, 

 and in consequent variation of this part of the body. 



As sterile insects are simply sexual insects which have 

 not become perfectly developed, we must believe that all 

 their characteristics are shared by the sexual insects, 

 and there is therefore no great difficulty in understand- 

 ing how the action of unfavorable conditions upon the 

 sexual form might cause variation in the sterile form. 



The various cells of the body stand in such intimate 

 relations to each other, and are mutually dependent 

 upon each other in so many ways, that it is quite impos- 



