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. IV. The question as to whether any advantage is gained by having each new individual produced by the union of two germ-cells, or by having the germ-cells carried by two kinds of individuals. While our main problem is concerned with the last of these topics, yet there would be little hope of giving a complete answer to it unless we could get some answer to the first three questions. The Different Kinds of Sexual Individuals Amongst the unicellular animals and plants the fusion of two (or more) individuals into a single one is generally regarded as the simplest, and possibly also the most primitive, method of sexual reproduction. Two amœbas, or amœba-like bodies, thus flow together, as it were, to produce a new individual. In the more highly specialized unicellular animals, the processes are different. Thus in vorticella, a small, active individual unites with a larger fixed individual. The protoplasm fuses into a common mass, and a very complicated series of changes is passed through by the nucleus. In paramœcium, a free-swimming form very much like vorticella, two individuals that are alike unite only temporarily, and after an interchange of nuclear material they separate.