THE THEORY OF NATURAL SELECTION. 283 



on the moisture. A plant which annually produces a 

 thousand seeds, of which only one on an average comes 

 to maturity, may be more truly said to struggle with 

 the plants of the same and other kinds which already 

 clothe the ground. The mistletoe is dependent on the 

 apple and a few other trees, but can only in a far-fetched 

 sense be said to struggle with these trees, for, if too 

 many of these parasites grow on the same tree, it 

 languishes and dies. But several seedling mistletoes, 

 growing close together on the same branch, may more 

 truly be said to struggle with each other. As the 

 mistletoe is disseminated by birds, its existence depends 

 on them; and it may metaphorically be said to struggle 

 with other fruit-bearing plants, in tempting the birds to 

 devour and thus disseminate its seeds.' 



(3) The third proposition of the theory of natural 

 selection is that all living beings are subject to variation. 

 As has been previously seen, the individuals which com- 

 pose any and every 'species' of animals and plants are 

 not precisely alike. They invariably differ from one 

 another in more or less numerous points, some of the 

 differences being extremely minute, while others may be 

 very conspicuous. We do not know whether variation 

 is indefinite, and affects every part of the organism, or 

 whether it is definite and is confined within certain limits. 

 Nor has it been clearly proved whether variation is 

 fortuitous, or whether it takes place in obedience to 

 some determinate law, which governs the direction which 

 it follows. It is, however, certain that ( variation,' to a 

 greater or less extent, is of universal occurrence among 

 all living beings. 



