THE LIVING MACHINE BUILDING FACTORS. 187 



all higher animals and plants at least, each indi- 

 vidual has two parents instead of one. In our 

 study of cells we have seen that the machinery 

 of the cell is such that it requires in the ordinary 

 process of reproduction the union of germinal 

 material from two different individuals to pro- 

 duce a cell which can develop into a new indi- 

 vidual. As we have seen, the egg gets rid of 

 half its chromosomes in order to receive an equal 

 number from a male parent; and thus the fer- 

 tilized egg contains chromosomes, and hence 

 hereditary material, from two different indi- 

 viduals. Now, this sexual reproduction occurs 

 very widely in the organic world. Among some 

 of the lowest forms of unicellular organisms it 

 is not known, but in most others some form of 

 such union is universal. Now, here is plainly an 

 abundant opportunity for congenital variations ; 

 for it is seen that each individual does not come 

 from germ material identical with that from which 

 either parent came, but from some of this material 

 mixed with a similar amount from a different parent. 

 Now, the two parents are never exactly alike, 

 and hence the germ plasm which each contri- 

 butes to the offspring will not be exactly alike. 

 The offspring will thus be the result of the un- 

 folding of a bit of germ plasm which will be 

 different from that from which either of its parents 

 developed, and these differences will result in 

 congenital variations. Sexual reproduction thus 

 results in congenital variations ; and if congenital 

 variations are necessary for the evolution of the 

 living machine and we shall soon see reason for 

 believing that they are we find that sexual 



