630 ZOOLOGY SECT 



The plant must find a place with soil containing certain constituents, 

 and with 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 seedling 

 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 seedling 

 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 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 unenclosed 

 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 

 concerned, 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 ssnse, 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 indiscrimi- 



