THE LIVING MACHINE BUILDING FACTORS. 157 



herits from its parents, but why it is like its 

 parents. While biologists are still in dispute 

 over many problems connected with heredity, 

 all are agreed to-day that this principle of the 

 continuity of the heredity substance must be the 

 basis of all attempts to understand the machinery 

 of heredity. But plainly this whole process is a 

 function of the cell machinery. While, therefore, 

 the idea of the continuity of germ substance 

 greatly simplifies our problem, we must acknowl- 

 edge that once more we are thrown back upon 

 the mysteries of the cell. Until we can more 

 fully explain the cell machine we must recognize 

 our inability to solve the fundamental question 

 of why an individual is like its parents. 



But plainly reproduction and heredity, as we 

 have thus far considered them, will be unable to 

 account for the slow modification of the machine; 

 for in accordance with the facts thus far outlined, 

 each generation would be precisely like the last, and 

 there would be no chance for development and 

 change from generation to generation. If the 

 individual is simply the unfolding of the powers 

 possessed by a bit of germ plasm, and if this 

 germ plasm is simply handed on from generation 

 to generation, the successive generations must of 

 necessity be identical. But the living machine 

 has been built by changes in the successive gen- 

 eration, and hence plainly some other factor is 

 needed. This factor is variation. 



Variation, Variation is the principle that pro- 

 duces modification of type. Heredity, as just ex- 

 plained, would make all generations alike. But 

 nothing is more certain than that they are not 

 alike. The fact of variation is patent on every 

 side x for no two individuals are alike. Successive 



