THE PROBLEM OF UTILITY 381 



Why Species are Stable 



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 part 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 fluctu- 

 ating mass of variable units which yet maintain the same 

 general average or 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 

 approximately fixed. The species is maintained in harmony 

 with its environment by the survival of the fittest. 



Why Species become Modified. 



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 affects the species. It must, therefore, undergo 

 some amount of modification, either structural or functional, 

 in order to succeed under the new conditions ; and the 

 constant variations of every part (around its mean) furnish 

 the materials for adapting the organism to these new condi- 

 tions. If a new enemy is the danger to be guarded against 



