XV THE PHILOSOPHY OP ZOOLOGY 051 



sides. To begin with, before there is aay struggle for existence in 

 the strict sense, there is — particularly in lower groups — a very great 

 indiscriminate destruction of ova and young embryos. Most lower 

 animals produce ova in great number — hundreds, more often 

 thousands and tens of thousands, annually. Only a few of these 

 reach maturity ; a large proportion are destroyed indiscriminately 

 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 

 kind, 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 

 ia 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 Oruciferoe — 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 

 di£ferences 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 



