THE PKOBLEM OF UTILITT. 483 



quite completed. But in the great majority of species definite 

 characters do exist by which any single individual can be 

 recognized and the species to which it belongs be determined ; 

 and the question is, whether or no the characters, or combination 

 of characters, which thus differentiate it are now useful or were 

 useful at the time of its origination*. In order to answer this 

 question, we must briefly summarize both the facts and the 

 admitted principles or theories which bear upon it. 



Every extensive area contains a number of large and dominant 

 species which appear to be, and probably are for considerable 

 periods, stable, both in average population and in the extent of 

 the area they occupy. Taking any one of these species — say 

 of bird or mammal — so long as the whole conditions of its 

 environment remain unchanged or very little changed it; will, 

 theoretically, continue to maintain itself, as we know many 

 species have maintained themselves during the whole period 

 since the glacial epoch, and some very much longer. The 

 species, however, is not absolutely homogeneous. It varies in 

 every generation, not minutely or infinitesimally as was formerly 

 supposed, but very considerably, the variations being easily seen 

 and measured by any one who looks for them ; and they extend, 

 so far as we know, to every })art of the organism, external and 

 internal, since no part has yet been found to be invariable when 

 a large number of individuals have been compared. The species 

 is therefore composed of a fluctuating mass of variable unics 

 which yet maintain the same general average of characters, and 

 this it can only do by a constant or intermittent weeding out of 

 the extremes in every direction. Such a weeding out on a large 

 scale takes place annually, because, although the annual increase 

 by birth is very large, the population of adults remains approxi- 

 mately fixed. The species is maintained in harmony with its 

 environment by the survival of the fittest. 



But now let some important change occur, either in climate, 

 in abundance of food, or by the irruption of some new and 

 hitherto unknown enemies, a change which at first injuriously 



* To this should be added — " or were correlated with some useful characters." 

 I have referred to such correlations in my ' Natural Selection and Tropical 

 Nature,' pp. 172 and 175 ; and as to apparently useless characters being in 

 some cases correlated with those which are useful, in uiy ' Darwinism,' p. 140 ; 

 but it is cumbersome to restate this part of the theory whenever it is stated 

 that all specific characters are useful. 



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