xv THE PHILOSOPHY OF ZOOLOGY 615 



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 a,s 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 all 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 

 have been produced to a great extent under domestication. These 

 are not all mere superficial differences, but involve also the 

 proportions and shape of the parts of the skeleton. The difference 

 in the form of the skull and in the proportions of the bones of the 

 limbs between a Greyhound and a Bulldog, for example, are very 

 remarkable so great, in fact, that if they were found to occur 

 between two wild forms they would justify a zoologist in referring 

 the two to distinct genera. Sheep and Cattle, Pigs and Horses 

 present similar, though not perhaps quite so strongly-markedj 

 varieties. One of the most remarkable cases of variation under 



