Natural selection works thus apparently for the survival of the individuals possessing in the aggregate the most complete conformity to environment.  Let us now imagine that an animal is so constructed as to be capable of variation along several disadvantageous or neutral lines, and along only one which is advantageous.  The development would of course proceed along the advantageous line.  Let us farther imagine that to the descendants of this individual two, and only two, advantageous lines of variations are allowed by its structure.  Then natural selection would probably favor the decidedly advantageous line, if such there were.  But as long as the structure of the animal allows variation along only a few lines, the two advantageous variations would, according to the law of probabilities, frequently occur in the same individual.  The eggs and spermatozoa of two such individuals might not infrequently unite, and thus in time the two characteristics be inherited by a large fraction of the species.

And now let me quote from Mr. Spencer: 

“But in proportion as the life grows complex—­in proportion as a healthy existence cannot be secured by a large endowment of some one power, but demands many powers; in the same proportion do there arise obstacles to the increase of any particular power, by ‘the preservation of favored races in the struggle for life.’  As fast as the faculties are multiplied, so fast does it become possible for the several members of a species to have various kinds of superiorities over one another.  While one saves its life by higher speed, another does the like by clearer vision, another by keener scent, another by quicker hearing, another by greater strength, another by unusual power of enduring cold or hunger, another by special sagacity, another by special timidity, another by special courage; and others by other bodily and mental attributes.  Now it is unquestionably true that, other things equal, each of these attributes, giving its possessor an extra chance of life, is likely to be transmitted to posterity.  But there seems no reason to suppose that it will be increased in subsequent generations by natural selection.  That it may be thus increased, the individuals not possessing more than average endowments of it must be more frequently killed off than individuals highly endowed with it; and this can happen only when the attribute is one of greater importance, for the time being, than most of the other attributes.  If those members of the species which have but ordinary shares of it, nevertheless survive by virtue of other superiorities which they severally possess, then it is not easy to see how this particular attribute can be developed by natural selection in subsequent generations.  The probability seems rather to be that, by gamogenesis, this extra endowment will, on the average, be diminished in posterity—­just serving in the long run to compensate the deficient endowments of other individuals whose special powers