produce a human being. The great mass of the cells of this individual, the body or somatic cells, take no part whatsoever in giving rise to the germ-cells of the next generation. These are produced from the pre-existing germ-cells, and from no other source, and it is for this reason alone that the phenomena of heredity are possible and that one generation is directly continuous with its predecessors. In fact heredity may be defined as the relationship which exists between successive generations.

We therefore see that the embryo, or the individual, is formed from one, and one only, of the first products of the division of the fertilised germ-cell, the rest of these products forming the other body tissues. This idea of the continuity of the germ-plasm is the greatest contribution of modern embryological research. It is quite fundamental, and no clear understanding of what is involved in the making of a man is possible without it. It teaches us that the line of ancestry and heredity is from one generation of germ-cells to another, directly, and never through the individual from the embryo, which, indeed, is a mere side product in the continued chain of events. The individual is practically the trustee of the germ-cells, but not their maker. No embryo, and no individual, ever made germ-cells; the latter existed first. The object of the embryo is obviously to protect and nourish the germ-cells which have been placed within it, so that they may be available in due time for the production of further germ-cells, and so for the continuity of the race. Hence it is the all-pervading truth of natural selection that the interest and survival of the individual is almost of no account; that of the species or the race being the paramount consideration.

Once these facts be grasped there is no longer any difficulty in understanding why the process of reproduction in any given species always results in the formation of embryos which resemble each other in all the main characters of their species. It could not be otherwise, because they come from portions of identically similar material, a common germ-plasm. In other words, the individual inherits nothing from its parents. He merely receives in his turn the material inheritance in the germ-plasm which was there a generation before him.

In so far as there has been no germinal variation he and they will be similar. Hence the common observation that the child resembles the parent. True, so he does; but not because he gets his characters from them, but simply because he and they obtain their characteristics from a common source. To many this thought will be, perhaps, a new one. It is one of the most interpreting ideas which science has given us, and in its absence no real grasp of the origin of the physical, mental, and other characters if there be any, of the embryo can be understood. The present writer has elsewhere summarised this thought as follows:—

“Man is composed partly of characteristics, which are derived from pre-existing germ-cells, and over the possession of which he has no control whatsoever. Be they good, bad, or indifferent, these characteristics are his from his ancestry in virtue of his inheritance. The possession of these characteristics is to him a matter of neither blame nor praise, but of necessity. They are inevitable.”

The embryo then which is to form the individual starts its career with a certain number of innate germinal characteristics which manifest themselves in the form of tendencies to grow in this direction or that. During the period of gestation a good many of these tendencies are well developed while a good many more only manifest their exact nature in later life. But it is upon the basis of these tendencies—and upon no other—that the making of the individual is possible. They represent the total assets available for the formation of character. Nothing of any new kind can be added to them.

All that can be done under the best conceivable circumstances is so to arrange the environment and surroundings of the embryo, and the subsequent individual, so that these tendencies are acted upon in such a way that the best are developed, and the worst eliminated. It must be remembered that it is under the constant action of everything that constitutes the environment of an embryo that the mass of body-cells gradually grows into a recognisable human personality.

The question then arises, What are the factors external to the embryo which cause these germinal tendencies to become active and fully developed? These factors are those of (a) nourishment; (b) use, or exercise; and (c) injury. In the case of the human embryo by far the most important of these three factors is the first. A proper supply of nourishment and food, that is to say maternal nutrition of adequate quantity, is sufficient up to the time of birth to cause the inborn tendencies in all the body-cells gradually to assume the special characteristics of muscle, bone, gland, nerve, and so forth, which make the human embryo. After the period of embryonic life is over, the stimulus of nutrition is still sufficient for some of these body-cells. Thus we find that the hair, the teeth, the internal ears, and the organs of reproduction, all grow to their full development in the absence of any other factor or stimulus than that of nutrition. But, as we also know, this simple stimulus is not sufficient for most of the other body tissues to develop properly. They require the additional stimulus of exercise which, indeed, may be said to begin even in the life of the embryo. After that it is quite hopeless to expect a healthy embryo to develop into a fine child unless to the stimulus of nutrition there is added that of exercise. It is from the varying quantities and qualities of the three factors of nourishment, exercise, and injury, that part of the explanation is found for the variation in individuals of the same family. Starting with a good many of the same inborn tendencies none of them afterwards receive quite the same kind and amount of these stimuli, under the action of which they develop. And so we reach the second point, namely, that, in addition to innate characters certain others are subsequently acquired by the embryo of the individual in response to particular stimuli acting from without.

Here we are upon ground which is more or less in our own choice or control. It is impossible to alter germ-plasm; but it is not impossible to control the environment in which it exists. To these two groups of characters, the germinal and those acquired under stimulus, there is to be added the third group which we have mentioned on a previous page, namely, those that are usually termed variations. For example, one occasionally finds that one individual in a family, the parents of which, and the other members of which, are quite normal, may be born with six fingers instead of five. Similarly one of a family may have a variation in the direction of an extraordinary capacity for the acquisition of knowledge of certain types. Hence the genius in music, mathematics, memory, morality, and so forth. As we have seen, these variations are termed spontaneous, to express the fact that we are at present ignorant of the laws in accordance with which they arise, though, of course, it is understood that those laws must exist.

We have now surveyed the whole field of the possible origin of the characters of an embryo, and these may be summed up in the following tabular statement.