xv THE PHILOSOPHY OF ZOOLOGY 631 



nately at one stage or another of their development, some failing to 



reach a spot favourable for their development, others becoming 



the food of other animals. But such of the young as are less 



adapted to escape the various dangers to be encountered, and less 



fitted to procure the necessary food, are more likely to be destroyed. 



This is one phase and the most important, perhaps, of all of the 



struggle for existence among animals. But there is also a struggle 



for existence not only between individual animals of the same 



kii'd, but between animals of different kinds. This struggle, in so 



far as it relates to the competition for food and shelter, is more 



severe between nearly-related species ; for in such a case the food 



and the favourable conditions required are the same, or nearly so, 



in the two competitors. But there is also a struggle for existence 



of a constant and severe kind which goes on between carnivorous 



animals and the animals on which they prey a struggle in which 



the defensive qualities of the latter, such as swiftness, power of 



eluding observation, power of resisting attack and the like, are 



opposed to the predatory powers of the former. 



Variation. It was by observing this struggle for existence 

 constantly going on in nature, taken in connection with the 

 phenomenon of variation, that Darwin was led to his principle of 

 Natural Selection as accounting for evolution. Variations in 

 domestic animals and cultivated plants are observed to take place 

 in various directions. Taking advantage of this, man has been able 

 to select, in the animals which he has domesticated and the plants 

 which he has cultivated, those qualities which seemed most likely 

 to be useful to him ; he has thus been able to produce, from one 

 and the same original wild stock, widely different varieties specially 

 adapted for different purposes. Thus from one wild species of plant 

 of the order Cruciferce viz., Brassica oleracea have apparently 

 been produced all the varieties of cabbage, cauliflower, broccoli, 

 Brussels sprouts, and other forms, each with a peculiar and strongly 

 marked growth of its own. All the domestic vegetables afford us 

 instances of the same thing, and so do all the cultivated fruits. 

 The crab-apple or wild apple, for example, was the original of all 

 the varieties of apple, amounting to about a thousand, cultivated 

 at the present day varieties presenting in many cases very great 

 differences in size, colour, texture, flavour, time of ripening, and 

 other qualities. In cultivated flowers the same holds good in an 

 even higher degree. 



The instances of variation observable among domestic animals 

 are still more striking. The domestic Dog, for example, exhibits a 

 large number of very marked varieties. Though all these seem to 

 be fertile with one another, and to produce fertile offspring, it is 

 generally supposed that they have been derived from several wild 

 species with more or less hybridisation. But the enormous 

 differences which are to be observed between some of the varieties 



