1888.] Evolution as illustrated by the Geogr. Distrib. of Plants. 281 



will be cross-fertilized from A's, and about au equal number of the A's 

 from the B's. Of these inheriting on one side the B form and on the 

 other the tendency to the same form, the great majority will be of the B 

 form, while of the A's crossed with A's, another 20 per cent will pass 

 over to the B form. In this way it is obvious that the A form will 

 soon begin rapidly to give way to the B, and subsequent forms 

 tending rearer and nearer to X. We have here been supposing that 

 natural selection does not operate, and that external circumstances are 

 equally favourable to A B and the subsequent forms up to X. If, 

 however, this be not so, if natural selection favours the development 

 towards X, then the evolution of the X species and elimination of the 

 A species will be facilitated. If, however, natural selection be opposed 

 to the B variation, then the A form will simply tend to die out in its 

 attempt to vary in a way in which it is not permitted to vary. 



As this tendency to vary is supposed to characterize the species 

 wherever it exists, it is easy in this way to account for the compara- 

 tively rapid extinction of a species which is actively varying. 



But it may be asked what proof have we of the existence of this 

 capacity to vary in comparatively few directions and tendency to 

 vary in fewer still. 



We cannot say that we have any distinct proof of the existence 

 of such laws except in so far as these laws better explain the 

 evolution and extinction of species. Some arguments however may 

 be urged in their support : 



(1.) The doctrine of the correlation of organic variation. 



This doctrine is not to be confounded with that of the correlation 

 of organs. 



Whatever be our theory as to the origin of species or the cause 

 of variation, it is plain that no plant or animal could exist unless 

 its organs were correlated not only with each other but with the 

 environment. Their eyes, teeth, limbs and digestive organs are 

 correlated with each other and the food supply. In order that 

 varieties may subsist they must involve the correlative variation of 

 the necessary organs. The harmony of life must be maintained. 

 Variations opposed to this harmony will be disadvantageous to the 

 individual. 



The bare theory of casual variation with natural selection meets 

 this requirement by supposing that among the infinite varieties which 

 are always occurring those only will survive which involve the 

 requisite harmony with each other and the environment. 



