made one species adapted for the plain, another for the mountain forest were _already there_; they can only be referred to personal selection, which brought about the adaptation of their ancestors in the course of ages to the conditions of their life. Something similar is true of the adaptations of the tissues; the differentiation of the individual kinds of cells is an ancient inheritance, and depends upon personal selection, but their distribution and arrangement into specially adapted tissues, so far as there is any plasticity at all, depends upon histonal selection. Obviously, however, only as far as the tissue is plastic, that is, with the power of adjusting itself to particular local conditions. Only adaptations of this kind can be referred to histonal selection; the ground-plan, even of the most complicated tissue, such as the large glands of mammals, the kidneys, the liver, and so on, must have been implicit in the germ, and must therefore be referred to personal selection. A precise limitation of the respective spheres of action of personal selection and histonal selection is not possible as yet, since hardly any investigations on the subject are available. Roux undoubtedly over-estimated the influence of his 'struggle of parts' when he believed that the most delicate adaptations of the different kinds of cells depended on it. I admit that, for a considerable time, I made the same mistake, until it became clear to me, as it did first in regard to the sex-cells, that this is not, and cannot be the case. How, for instance, could the diverse and minutely detailed adaptations of the sex-cells--which we are to discuss in a subsequent lecture--have arisen in this way? As far as the individual sperm-cell is concerned, it is a matter of indifference whether its head is a little thinner or thicker, its point a little sharper or blunter, its tail a little stronger or weaker. This does not decide whether the cell is to thrive better, or to occur in greater numbers than some other variety. But it does decide whether it is to be able to penetrate through the minute micropyle, or through the firm egg-envelope, into the egg, there to effect fertilization. An individual with less well formed sperm-cells will be able to fertilize fewer eggs, and therefore to leave fewer descendants which might inherit its tendency to produce inferior sperm-cells, and conversely. Thus it is not the sperm-cells of any one individual which are selected according to their fitness, it is the individuals themselves which compete with one another in the production of germ-cells which shall fertilize best, that is, most certainly. The struggle is thus not intercellular, but a struggle between persons.