THE LIVING MACHINE BUILDING FACTORS. 181 



But plainly reproduction and heredity, as we 

 have thus far cousidered 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 genera- 

 tions must of necessity be identical. But the 

 living machine has been built by changes in the 

 successive generation, and hence plainly some 

 other factor is needed. This factor is variation. 



VARIATION. 



Variation is the principle that produces modifi- 

 cation of type. Heredity, as just explained, 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, for no two 

 individuals are alike. Successive generations 

 differ from each other in one respect or another. 

 Birds vary in the length of their bills or toes ; 

 butterflies, in their colours; dogs, in their size 

 and shape and markings; and so on through 

 an endless category. Plants and animals alike 

 throughout nature show variations in the greatest 

 profusion. It is these variations which must 

 furnish us with the foundation of the changes 

 which have gradually built up the living 

 machine. 



