110 DARWINISM 



by rivals, or is in process of extinction by enemies, to save 

 itself by adopting new habits or by occupying vacant places 

 in nature. This is the immediate and obvious effect of all 

 the numerous examples of divergence of character which we 

 have pointed out. But there is another and less obvious 

 result, which is, that the greater the diversity in the organisms 

 inhabiting a country or district the greater will be the total 

 amount of life that can be supported there. Hence the 

 continued action of the struggle for existence will tend to 

 bring about more and more diversity in each area, which may 

 be shown to be the case by several kinds of evidence. As an 

 example, a piece of turf, three feet by four in size, was found 

 by Mr. Darwin to contain twenty species of plants, and these 

 twenty species belonged to eighteen genera and to eight 

 orders, showing how greatly they differed from each other. 

 Farmers find that a greater quantity of hay is obtained from 

 ground sown with a variety of genera of grasses, clover, 

 etc., than from similar land sown with one or two species 

 only ; and the same principle applies to rotation of crops, 

 plants differing very widely from each other giving the 

 best results. So, in small and uniform islands, and in 

 small ponds of fresh water, the plants and insects, though 

 few in number, are found to be wonderfully varied in 

 character. 



The same principle is seen in the naturalisation of plants 

 and animals by man's agency in distant lands, for the species 

 that thrive best and establish themselves permanently are 

 not only very varied among themselves but differ greatly from 

 the native inhabitants. Thus, in the Northern United States 

 there are, according to Dr. Asa Gray, 260 naturalised flower- 

 ing plants which belong to no less than 162 genera; and of 

 these, 100 genera are not natives of the United States. So, in 

 Australia, the rabbit, though totally unlike any native animal, 

 has increased so much that it probably outnumbers in in- 

 dividuals all the native mammals of the country ; and in 

 New Zealand the rabbit and the pig have equally multiplied. 

 Darwin remarks that this "advantage of diversification of 

 structure in the inhabitants of the same region is, in fact, the 

 same as that of the physiological division of labour in the 

 organs of the same body. No physiologist doubts that a 



