THE PAST HISTORY OF PLANTS. 203 



CHAPTER XIV. 



THE PAST HISTORY OF PLANTS. 



I PROMISED some time since to return in due 

 season to the question why plants, as a rule, ex- 

 hibit distinct kinds or species^ instead of merging 

 gradually one into another by imperceptible de- 

 grees. This problem is generally known as the 

 problem of the origin of species. You might per- 

 haps expect (since plants have grown and de- 

 veloped, 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 dis- 

 tinct kinds : and the reason is, that natural selec- 

 tion acts on the whole in the opposite 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 circumstances. 

 This form may perhaps vary again in other situ- 

 ations, and give rise to individuals better adapted 

 to the second set of circumstances. But just in 

 proportion as such individuals surpass in adap- 

 tation one another will they live down the less 

 adapted. Hence, the intermediate forms will 

 tend to perish, and the world to be filled in the 

 end with groups of plants, each distinct from 

 others, and each relatively fixed and similar 

 within its own limits. 



