animals at such times will trample and gore the distressed Due to Seatt In the case if wines i cer savagetempered caminos genes the stressed felow is frequently torn to partes and devoured on the spot. If any be brud enough to dam in this use that this habit Bas been angered because of advantage to the pack, in fa be mapped that the park gats more by feeding ch a weak member than by larg - take his chances of recovery, it may be pointed out in reply that cattle also destroy their weak or injured but do not devour them, and the same statement bilds for birds where the same instinct nas often been observed Rimates his suggested that the Instion of destroying the weak or injured members is of use because such members are a source of danger to the rest of the hard but Hudson points out that it is not so much the weak and Body members of the herd that are attacked in this way, as those that are injured and concludes the instinct is not only useless, but actually detrimental" He suggests that these wild abnormal movements of social animals" are a sort of aberration, so that in turning against a distressed Allow they oppose themselves to the law of being." Yet whether we gan anything by calling this action aberrant or abnormal, the important fact remains that it is a definite response under certain external conditions, and is shown by a. the individuals of the species.

T

The preseding Illustrations of reactions that go to make in the so-called instincts of animals may be separated into

hat are essential to the life of the individual or of the those that are of some apparent use, although not olitely essential, and a few of no use at all, and fewer that appear to be even injurious. If the latter reactions tave place only rarely, as appears often to be the case,

y are not sufficiently harmful to cause the destruction of the race. The evidence prints 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.

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 amoebas, or amœbalike 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.

In the lower plants, and more especially in some of the simple aggregates or colonial forms, there are found a number of stages between species in which the uniting individuals are alike, and those in which they are different. There are several species whose individuals appear to be exactly alike; and other species in which the only apparent difference between the individuals that fuse together is one of size; and still other species in which there are larger resting or passive individuals, and smaller active individuals that unite with the larger ones. In several of the higher groups, including the green algae and seaweeds, we find similar series, which give evidence of having arisen independently of each other. If we are really justified in arranging the members of these groups in series, beginning with the simpler cases and ending with those showing a complete differentiation into two kinds of germ-cells, we seem to get some light as to the way in which the change has come about. It should not be forgotten, however, that it does not follow because we can arrange such a series without any large gaps in its continuity, that the more complex conditions have been gradually formed in exactly this way from the simplest conditions.

So far we have spoken mainly of those cases in which the forms are unicellular, or of many-celled species in which all the cells of the individual resolve themselves into one or the

other kind of germ-cells. This occurs, however, only in the lowest forms. A step higher we find that only a part of the cells of the colony are set aside for purposes of reproduction. The cells surrounding these germ-cells may form distinct organs, which may show certain differences according to whether they contain male or female germ-cells. When these two kinds of cells are produced by two separate individuals, the individuals themselves may be different in other parts of the body, as well as in the reproductive organs.

When this condition is reached, we have individuals that we call males and females, because, although they do not themselves unite to form new individuals, they produce one or the other kind of germ-cell. It is the germ-cells alone that now combine to form the new individual.

Amongst living groups of animals we find no such complete series of forms as exist in plants, and the transition from the one-celled to the many-celled forms is also more abrupt. On the other hand, we find an astonishing variety of ways in which the reproduction is accomplished, and several ways in which the germ-cells are carried by the sexual individuals. Let us examine some of the more typical conditions under the following headings: (1) sexes separate; (2) sexes united in the same individual; (3) parthenogenetic forms; (4) exceptional methods of propagation.

1. Sexes Separate; Unisexual Forms.1- Although the animals with which we are more familiar have the sexes separate, this is far from being universal amongst animals and plants; and, in fact, can scarcely be said to be even the rule. When the sexes are separate they may be externally alike, and this is especially true for those species that do not unite, but set free their eggs and spermatozoa in the water, as fish, frogs, corals, starfish, jellyfish, and many other forms. In other animals there are sometimes other secondary differ

1 Geddes and Thompson's "The Evolution of Sex " has been freely used in the preparation of this part of this chapter.