156 Heredity. 



individuuls^ and the corresponding cell will therefore be 

 a hybrid, and will tend to vary in many descendants. 



In each of these descendants this hybrid will be com- 

 posed of almost identical elements, and tliey will all tend 

 to vary in the same or nearly the same manner; and as 

 each variation causes other cells to throw olf gemmules, 

 the number of individuals which are simihirly modified 

 will tend to increase from generation to generation, 

 and natural selection will therefore act, not on a single 

 exceptional individual, but upon a great number, all of 

 which are modified in essentially the same way. 



If Variation is Purely Fortuitous, tlie Evolution of a 

 Complicated Organ composed of Many Parts hy Nat- 

 ural Selection demands a Period of Time wliicli is 

 almost Infinite. 



This obvious objection to the law of natural selec- 

 tion has been so frequently discussed that it is un- 

 necessary to dwell upon it at present, especially as I shall 

 examine it in detail in another place. At present I will 

 only call attention to the fact that a variation in any part of 

 a complicated organ will, in itself, disturb the harmonious 

 adjustment of other parts, and will thus cause them to 

 throw off gemmules, and thus to induce variability in 

 the next generation. 



The fact that change is needed in any part will be the 

 cause of variation in this part, and the time which is 

 needed to restore all parts of an organ to a position of 

 equilibrium will thus be almost infinitely reduced. The 

 argument of those who hold that life has not existed 

 upon the earth long enough for the evolution of all the 

 adaptations of nature by the selection of fortuitous 

 yariations will thus lose all its weight. 



