Page:Popular Science Monthly Volume 68.djvu/487

Rh naturalist or systematist to determine which of these characters represent 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 position 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. Doubtless favorable fluctuations could be added together until those possessing them should be segregated from the mass about them, but we have no certain knowledge of any such cases. Doubtless individual fluctuations (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 variations 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 wellbeing 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