VARIATIONS IN ANIMALS AND PLANTS 483 



naturalist or systematist to determine which of these characters repre- 

 sent individual variations and which are associated with distinctness of 

 species. 



In general, the conditions of a species may be compared to a target 

 filled with marks of shot. The bull's-eye represents the normal posi- 

 tion of the shot, but variations in every direction occur. As one goes 

 away from the bull's-eye the shot marks are fewer in number, but 

 some of them may fall at a considerable distance from the center or 

 average. Sometimes the majority may lie on one side of the center. 

 This would indicate a continuous influence acting in one direction, 

 as the wind affecting the direction of the bullets. Among animals a 

 similar deviation from the usual may indicate a climatic effect, or the 

 effect of natural selection under different conditions. That either of 

 these influences may cause a mass variation within a group admits of 

 no shadow of doubt. Nor is there any doubt that fluctuations within 

 the species may and do constantly go as far as to equal the usual 

 distinction between one species and another. The sole test of a species 

 is in the relative permanence of its group of characters, not in the 

 extent of its deviations from some other group. And in almost every 

 case where the actual origin or cause of separation of one species from 

 the next can be traced, it is found to have a geographic factor. Doubt- 

 less favorable fluctuations could be added together until those possess- 

 ing them should be segregated from the mass about them, but we have 

 no certain knowledge of any such cases. Doubtless individual fluctua- 

 tions (saltations) could be so extreme as at once to segregate their 

 possessor from all its neighbors. Doubtless new species might and 

 perhaps do originate in that way, but out of the hundreds of thousands 

 of species studied in nature, not one is certainly known to have had 

 such an origin. But let any group of individuals be separated from 

 their fellows, no matter how, in time the individuals remaining will 

 come to an equilibrium with a different center of variation or a different 

 arrangement of average characters from those possessed by the parent 

 stock. The characters of any species represent the equilibrium or 

 result attained by the forces of heredity and the operations of local 

 natural selection. 



We may again divide the traits of the fluctuations or minor varia- 

 tions of the individual into three classes, those useful, those indifferent 

 and those harmful. A useful variation may cause the survival of the 

 individual; a harmful variation may cause its destruction. 



In considering the traits of a species, these classes of characters are 

 reduced to two. Adaptive characters — those associated with the well- 

 being of the individual — and non-adaptive, or indifferent characters — 

 those which have no evident relation to utility. Characters positively 

 harmful are always eliminated so far as the species is concerned. 



In the actual work of the description of species, we recognize at 



