ON THE CAUSES OF VARIATION. 485 



than the indigenes or endemic forms. This is readily compre- 

 hended on two grounds : First, that species which liave, in the 

 course of time, experienced a greater struggle among themselves 

 in large areas, have an advantage over those in more limited 

 areas in which the struggle has been less intense ; secondly, that 

 species which have accommodated themselves to the changes in 

 life conditions which civilized man induces, have a great advantage 

 when, following man's migrations, they are brought into competi- 

 tion with species which have not yet been subjected to such con- 

 ditions. Again, no valid reason can be urged why, within a given 

 area, one species predominates over another in so far as mere 

 adaptation is concerned. The influences of environment alone 

 would tend to unify the fauna and flora of a given region. Theo- 

 retically, so far as climate and physical conditions are concerned, 

 there is no reason, through regions where these are uniform, why 

 a single animal should not prevail to the exclusion of all others, 

 providing it was vegetarian, or that the particular plant which 

 furnished food to such an animal should not prevail to the ex- 

 clusion of all others. The hickory and the blade of grass must 

 be considered equally adapted to the environment with the oak, 

 and so on all through the multifarious forms of both vegetal and 

 animal life : so that this diversity of form can best be explained 

 by some principle like natural selection, and by the interrelation 

 and interaction of organisms and the struggle between them for 

 existence. This is illustrated in many directions. To take a 

 striking example : no one doubts that if the larger carnivora of 

 Europe and Asia were introduced into Australia, the marsupials 

 would soon have to give way, and could survive only by the 

 acquisition of special functional modifications and larger intelli- 

 gence such as we find in our opossum. Yet it would be folly to 

 conclude that the marsupials are less well fitted to the physical 

 conditions which obtain in Australia than their introduced ex- 

 terminators. 



From what has preceded, we are, I think, justified in rejecting 

 the interpretations of both extremists as to the scope and mean- 

 ing of natural selection. It can not be debased to the mere 

 expression of the universally observed fact of variability ; yet it 

 must be restricted, because it not only implies something to be 

 selected, but its promulgator limits its scope to the selection of 

 something that is useful. As a philosophy it considers only pro- 

 cesses, and leaves remote origin and cause untouched. The fol- 

 lowing limitations are probably justified to-day, and will help to 

 more exact use of the term : 



1. It deals only with individual variation from whatever 

 cause, and should not be applied to simultaneous variation in 

 masses. 



