312 THE LIFE OF THE PLANT 



we have but to recall the pains taken by a farmer to 

 save his fields from being overrun with weeds, to realise 

 the struggle our cultivated plants have to maintain, and 

 how easily they would perish if left to themselves. 

 Hence the fact of the struggle for existence, as the out- 

 come of the law of geometrical progression in the 

 multiplication of organic beings, obvious as any mathe- 

 matical truth, is proved by direct experiment. This 

 struggle in the same logically inevitable way leads to 

 natural selection, i.e. to more and more perfect adapta- 

 tion, although it may be liable to escape our notice 

 in any single generation. If we take the testimony 

 of geology into account as to the almost immeasurable 

 space of time that has elapsed since organisms appeared 

 upon the earth, we shall readily agree that the process 

 of selection, acting with such inexorable severity and 

 during such a lengthy period of time, can fully account 

 for both the variety of organic forms and the perfection 

 of their adaptation. 



Thus the evolution of organic forms and their infallible 

 trend towards perfection may be considered as a neces- 

 sary logical outcome of the three fundamental properties 

 of organisms. These are the capacity for variation, 

 the capacity for transmitting variations to posterity, 

 i.e. heredity, and the capacity for multiplication, which 

 is invariably connected with reproduction. 



The capacity of organisms for variation is indisput- 

 able. We do not know of any two absolutely similar 

 beings. Yet the causes of variation, and the relation 

 which variation bears to selection, need some further 

 explanation. The primary cause which produces 

 changes in an organism must lie in the indirect or 

 immediate influence of their external conditions ; and 

 then comes the influence of secondary causes, such as 

 correlation in the development of parts, the exercise of 

 organs, and so on. In most cases it is, however, very 

 difficult to discover the connexion between a variation 



