io MENDELISM CHAP. 



encouragement to such work, for in spite of the labour 

 devoted to these experiments, the results offered but a 

 confused tangle of facts, contributing in no apparent way 

 to the solution of the problem for which they had been 

 undertaken. After half a century of experimental hy- 

 bridisation the determination of the relation of species 

 and varieties to one another seemed as remote as ever. 

 Then in 1859 came the Origin of Species, in which Darwin 

 presented to the world a consistent theory to account 

 for the manner in which one species might have arisen 

 from another by a process of gradual evolution. Briefly 

 put, that theory was as follows : In any species of plant 

 or animal the reproductive capacity tends to outrun the 

 available food supply, and the resulting competition leads 

 to an inevitable struggle for existence. Of all the individ- 

 uals born, only a portion, and that often a very small one, 

 can survive to produce offspring. According to Darwin's 

 theory, the nature of the surviving portion is not deter- 

 mined by chance alone. No two individuals of a species 

 are precisely alike, and among the variations that occur 

 some enable their possessors to cope more successfully 

 with the competitive conditions under which they exist. 

 In comparison with their less favoured brethren they 

 have a better chance of surviving in the struggle for 

 existence and consequently of leaving offspring. The 

 argument is completed by the further assumption of a 

 principle of heredity, in virtue of which offspring tend to 



