The preceding illustrations of reactions that go to make up the so-called instincts of animals may be separated into those that are essential to the life of the individual or of the race, those that are of some apparent use, although not absolutely essential, and a few of no use at all, and fewer still that appear to be even injurious. If the latter reactions take place only rarely, as appears often to be the case, they are not sufficiently harmful to cause the destruction of the race. The evidence points to the conclusion, I believe, that the origin of these tropisms and instincts cannot be accounted for on the ground of their benefit to the individual or to the race; and it does not seem reasonable to make up one explanation for the origin of those that are essential, and another for those that are of little use or even of no use at all.
From what has been already said more than once, while discussing each particular case, the simplest course appears to be in all instances to look upon these instincts as having appeared independently of the use to which they may be put, and not as having been built up by selection of the individual variations that happen to give an organism some advantage over its fellows in a life and death struggle. It appears reasonable to deal with the origin of tropisms and instincts in general in the same way as in dealing with structures; for, after all, the tropism is only the outcome of some material or structural basis in the organism.
No attempt has been made here to interpret the more complex reactions of the nervous system, for until we can get some insight into the meaning of the simpler processes, we are on safer ground in dealing with these first.
CHAPTER XII
SEX AS AN ADAPTATION
In what sense may the separation of all the individuals of a species into two kinds of individuals, male and female, be called an adaptation? Does any advantage result to the species that would not come from a non-sexual method of reproduction? Many attempts have been made to answer these questions, but with what success I shall now try to show.
There are four principal questions that must be considered:—
I. The different kinds of sexual individuals in the animal and plant kingdoms.
II. The historical question as to the evolution of separate sexes.
III. The factors that determine the sex in each individual developing from an egg.