THE FACTORS OF EVOLUTION 149 



Other strange phenomena in this problem of the correlation of 

 the parts are that dogs and cats with white coats and blue eyes 

 are always deaf, and cats with yellow, black, and white stripes are 

 always females. It is a subject in which there is much room for 

 investigation. 



But let us return to artificial selection. We have already 

 seen that three factors must co-operate in the production of a 

 new breed : (1) Variability ; (2) conscious selection by the breeder 

 of such variations towards a certain direction, and (3) the ability 

 of the parental organisms to transmit their physical and mental 

 qualities in a more or less perfect degree to their descendants. 

 These three factors are the sine qua non of artificial selection : if 

 only one of these conditions is absent the formation of a new race 

 is not possible. 



Darwin's daring doctrine which he formulated on the basis 

 of his experiments in artificial selection was this : Exactly the 

 same causes which we see at work in the formation of new 

 species and breeds of domestic animals effect in free Nature, too, 

 the transformation of the organic world and the formation of new 

 species. But instead of the artificial breeder there is another, 

 natural, mode of selection, the battle of the organisms for 

 the means of livelihood, the struggle for existence. But whilst 

 in artificial selection animals are frequently bred possessing 

 characteristics which are of advantage to man in one way or 

 another, but of injury to the animals themselves, sometimes to 

 such an extent that many domestic animals are to-day unable to 

 live in a free state without the protection of man, the struggle 

 for existence tends to select those individuals which are best 

 fitted for the conditions of their existence ; the gradual develop- 

 ment, adaptation, and perfection of the ^organic world must, 

 therefore, be a necessary consequence of natural selection. 



That heredity is as highly developed in animals living in a 

 free state as in domesticated animals requires no proof, for it is 

 notorious that the various animal parents produce offspring 

 which, with the exception of slight individual variations, are their 

 very images. 



Less obvious is the second essential condition, variability, 

 though accurate investigation will always succeed in demon- 



