6 Descent 



factors in the general process of variation. Ee- 

 peatedly Darwin tried to formulate these causes, 

 but the evidence available did not meet his re- 

 quirements. 



Quetelet's law of variation had not yet been 

 published. Mendel's claim of hereditary units 

 for the explanation of certain laws of hybrids 

 discovered by him, was not yet made. The 

 clear distinction between spontaneous and sud- 

 den changes, as compared with the ever-present 

 fluctuating variations, is only of late coming into 

 recognition by agriculturists. Innumerable 

 minor points which go to elucidate the breeders ' 

 experience, and with which we are now quite 

 familiar, were unknown in Darwin's time. No 

 wonder that he made mistakes, and laid stress 

 on modes of descent, which have since been 

 proved to be of minor importance or even of 

 doubtful validity. 



Notwithstanding all these apparently unsur- 

 mountable difficulties, Darwin discovered the 

 great principle which rules the evolution of or- 

 ganisms. It is the principle of natural selec- 

 tion. It is the sifting out of all organisms of 

 minor worth through the struggle for life. It 

 is only a sieve, and not a force of nature, not a 

 direct cause of improvement, as many of Dar- 

 win's adversaries, and unfortunately many of 

 his followers also, have so often asserted. It is 



