650 ZOOLOGY sect. 



and variation, both of which have to be understood before it 

 is possible to grasp the theory of natural selection. 



Struggle for Existence.— In order that it may flourish, there 

 are necessary for every species of plant and animal certain con- 

 ditions. The plant must find a place with soil containing, certain 

 constituents, and Avith a certain degree of moisture and of sunlight. 

 For spots presenting the necessary favourable conditions there is 

 constantly going on a competition between individual plants of 

 one species and between the members of different species. The 

 nature of this struggle is well seen when a piece of garden-ground 

 is allowed to run to waste. Its surface is soon overgrown by 

 weeds of a variety of kinds, which kill out some of the original 

 garden-plants. By and by the more hardy weeds kill out and 

 replace such weaker forms as may first have obtained a footing, 

 till an entirely new set of weeds may take the place of those that 

 first appeared. Again, it was shown by Darwin that in turf which 

 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 especially, sometimes prpduce 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 



