CH. XI] B. MORPHOLOGICAL 107 



green-fly is well known (42, p. 188) and even in the Podoste- 

 maceae, annuals starting again every year, one plant might in 

 four years cover about 100,000 square miles. The fiercest struggle 

 for existence comes to a plant at birth, and any that is not suited 

 to the conditions as they are at that moynent will be killed out by 

 natural selection by reason of unsuitability, though of course mere 

 chance will have a large influence in the matter. But this is an 

 individual struggle, and we have no right therefore to assume that 

 species struggle as units. Nothing can come into permanent 

 existence without the permission of natural selection, but once 

 the newcomer has become established in a few places reasonably 

 far apart, the chance of its being completely killed out will 

 steadily diminish, and in course of time may be reduced to 

 vanishing point. Natural selection simply determines in each 

 individual case whether or not a given plant shall be allowed to 

 survive and reproduce. 



Very few indeed of the morphological features that distinguish 

 one organism from another that is related to it have any physio- 

 logical significance at all, especially in those features that 

 separate the higher groups of plants from one another. Even the 

 bulk of the generic and specific characters come into the same 

 category. One cannot imagine any adaptational reason why 

 Ranunculus should have over 300 species, and world-wide distri- 

 bution, while its closest allies, like Myosurus or Oxygraphis, have 

 few species, are comparatively localised, and differ largely in the 

 fact that the wall of the fruit is not so hard. Still less can one 

 imagine adaptational reasons taking part in the separation of the 

 family Ranunculaceae into a group with achenes and another 

 with follicles, or one with alternate leaves and one with opposite. 

 Nor can one suggest adaptational reasons for the existence of 

 200 species of Clematis, and still less for that of a couple of 

 thousand Senecios or Astragali. If natural selection is to be held 

 responsible for the vast dispersal of, and numbers of species in 

 these genera, they must have some very great adaptational ad- 

 vantage over their close allies. And the adaptation which was so 

 successful must have been generic, for most of the species have 

 but small areas. There is no species in these very large and wide- 

 spread genera whose range covers that of the genus, though in 

 smaller and less widely dispersed genera this is very commonly the 

 case. 



It has frequently been shown, e.g. by de Vries (66, p. 224) and 

 by J. T. Cunningham and others, that adaptation shows chiefly 



