II HISTORICAL 9 



for which they had been undertaken. After half a 

 century of experimental hybridisation the determina- 

 tion 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 pre- 

 sented 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 individuals 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 determined 

 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, conse- 

 quently, of leaving offspring. The argument is 

 completed by the further assumption of a principle 

 of~ heredity, in virtue of which offspring tend to 

 resemble their parents more than other members 

 of the species. Parents possessing a favourable 

 variation tend to transmit that variation to their 

 offspring, to some in greater, to others in less degree. 

 Those possessing it in greater degree will again have 

 a better chance of survival, and will transmit the 

 favourable variation in even greater degree to 



