THE PAST HISTORY OF PLANTS 221 



its soil, its surroundings, its allies, its hired 

 friends, its blackmailing foes, its exterminating 

 enemies. Such exhaustive knowledge of the 

 tiniest weed is clearly impossible ; but even the 

 little episodes we can pick out piecemeal are full 

 of romance, of charm, and of novelty. 



CHAPTER XIV. 



THE PAST HISTOIIY OF PLANTS. 



I PHOMisED some time since to return in due 

 season to the question why plants, as a rule, 

 exhibit distinct kinds or species, instead of 

 merging gradually one into another by imper- 

 ceptible degrees. This problem is generally 

 known as the problem of the origin of species. 

 You might perhaps expect (since plants have 

 grown and developed, as we have seen, one out 

 of the other) that they would consist at present of 

 an unbroken series, each melting into each, from 

 the highest to the lowest. This, however, is 

 not really the case ; they form on the contrary 

 groups of distinct kinds : and the reason is, that 

 natural selection acts on the whole in the oppo- 

 site direction. It tends to make plants group 

 themselves into definite bodies or species, all 

 alike within the body, and well marked off from 

 all others outside it. 



Here is the way this arrangement comes 

 about. As situations and circumstances vary, 

 a form is at last arrived at in each situation 

 which approximately fits the particular circum- 



