614 ZOOLOGY 



SECT. 



is kept cut close a much greater number of plants are enabled to 

 grow than is the case if the turf is allowed to grow freely : If the 

 turf is not kept cut some of the stronger plants gain predominance 

 and kill out weaker forms. In a space of turf on which Darwin 

 experimented, no less than half of the species present in the turf 

 when kept pretty closely shaven perished when it was allowed to 

 grow freely. 



Plants, however, have not only to compete with one another for 

 space and light and nourishment. They have also numerous 

 animal foes to contend with. A large proportion of young seed- 

 ling plants are destroyed by various insects and by snails and slugs. 

 One of Darwin's experiments bearing on this' point was to clear 

 and dig up a small plot of ground and watch the fate of the seed- 

 ling plants that sprang up on it : he found as a result that some 

 four-fifths were destroyed by insects, snails, and slugs. But it 

 is not the lower forms of animals alone that are thus destructive 

 to plants. Many of the Mammalia, particularly, as we should 

 expect, the herbivorous Ungulata, exercise a strong influence in 

 this way. Cattle, and Goats particularly, sometimes produce a 

 marked effect on the flora of a country. The introduction of Goats 

 has been observed gradually to destroy the forests of certain 

 districts the seedling plants being eaten as they appear, and 

 thus no young trees being developed to take the place of those 

 dying from old age or other causes. The mere enclosing of a 

 piece of moorland by means of a fence was observed by Darwin to 

 have resulted in the growth of a number of trees. In the unen- 

 closed parts the young trees were never able to make any headway 

 against the cattle by which they were constantly being browsed 

 down. 



Among animals, with which we are here more particularly con- 

 cerned, as well as among plants, a struggle for existence goes on on all 

 sides. To begin with, before there is any 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 



