still some further considerations in connection with the subject of germ-cells and germ-plasm which we must carefully consider before leaving this part of our subject, Embryology. Everything depends upon a perfectly clear understanding at this stage. The facts themselves that have to be adduced in this connection are comparatively few and simple. No fairly educated person should have any difficulty whatsoever in grasping them. Moreover, very fortunately they are thoroughly well established and not in dispute. But the reasoning which is based upon these few and elementary facts, reasoning which is applied to the methods of treatment of the individual which is produced, may be very complicated and very debatable. Various schools of thought and opinion exist according to the attitude taken towards the facts, some of which we have mentioned and others of which we are about to detail. But the facts themselves are not debatable, and we therefore see once more that their importance at this stage cannot be exaggerated.

One or two very simple general propositions bound up with the subject of Embryology, or individual development, may be stated in order to focus attention upon the nature of the problem under investigation. Thus nobody will be found to question the fundamental truth that children resemble their parents. That is a commonplace of experience. Similarly no one will be found to dispute another fundamental fact, namely, that children differ from their parents. This, too, is equally a commonplace of experience. If we examine a million human beings we find that they all possess certain features in common, certain characteristics in virtue of which we recognize them to be human beings. Nevertheless it is just as true that a careful examination of the same million people reveals the true saying that no two of them are exactly alike. Here then are two propositions equally true within certain limits; namely, that all human beings resemble each other, and that all human beings differ from each other. There is resemblance; and there is variation. These two things are universal because of the existence and characteristics of germ-cells. We may look at this a little closer.

Every species of animal, in the process of reproduction brings forth offspring similar to itself. This is expressed in the familiar proverb that “like produces like.” One does not expect grapes from thorns, nor is it possible to construct a silk purse out of a sow's ear. But what is the explanation of this proverbial fact? The answer is of great importance, because although the fact itself is recognized as a general principle in the reproduction of a species, it is not sufficiently recognized in the full details of the characters of that individual. Too many people are still apt to expect to be able to produce grapes when the plant is a thorn, and it is unfortunately all too common to make heroic but quite futile attempts to construct human silk purses out of human sows' ears—so to speak—simply because of the ignorance of the material which is being used. The most that can be done is to give such material as is present the very best opportunity of attaining its own utmost perfection; and this, by the way, is vastly more than has ever been done for any considerable number of the human race.

But why this continuity of species? Why should like always produce like? The answer has been sought by biologists ever since problems of life attracted man's curiosity. All sorts of weird and fantastic theories have been put forward at different times to account for this simple fact, but it is only in comparatively recent years that the real explanation has been forthcoming. It is perfectly obvious that in order to secure this continuity of racial resemblance there must be something physical or material which is actually continuous from generation to generation to account for it. The immortal Darwin saw this very clearly, and devoted much thought in the endeavor to find some explanation of this very problem. The result was his theory of Pangenesis which, ingenious as it was, was ultimately shown to have no basis on fact. In his effort to account for the fact that children resemble their parents even in such minute details as the shape of the nose, the colour of the eyes, and so forth, he formulated the idea that the parents themselves probably contributed multitudes of minute particles from their own tissues to form the cells of their offspring. He supposed, for example, that particles or gemmules from the eyes, nose, hair, and so forth, of the parent, or parents, in some way or other were fused together and gave rise to the cells which ultimately produced an embryo. Hence he thought the explanation of the resemblance between parents and children. This was his solution to the question of the physical continuity between successive generations. It may be remarked in passing that it is with something of pathos that one reads in Darwin's own works his own evident opinion that this theory of Pangenesis was a great discovery. One gathers almost that he himself regarded it as of greater importance than his work on natural selection.

In the course of time, however, the real actual basis of physical continuity was shown to be something quite different, and looking back now upon the history of the discoveries in this connection during the last generation one can easily imagine what speculations there must have been in the absence of the facts which are now known to embryologists.

CHAPTER III

PROBLEMS OF REPRODUCTION (continued)

The one outstanding discovery which has placed the science of Embryology on an absolutely firm basis, and which has made clear so many of the facts, which were previously puzzling, is this: that the germ-cells which give rise to new individuals are themselves produced from pre-existing germ-cells. The entire embryo, or young infant, is derived from one single cell which we have called the fertilised ovum, and that in its turn was derived from the union of two germ-cells, one from the male parent, and one from the female. These two cells in their turn were also derived in a straight line of descent from the fertilised ovum from which each parent sprang. In other words there has never been any conjugation between one fertilised ovum and another in spite of the generations of cells which have been produced between them. Put in another way the body, or somatic cells, contribute absolutely nothing to the original material or germ-plasm of which the germ-cells are composed. They do not produce them in any sense of the word whatsoever, despite the popular opinion to the contrary. This is the great discovery of modern Embryology. Until this was known it was assumed that parents did produce the cells from which their children sprang, and hence—it was thought—the resemblance between them. The fact is quite otherwise. No parent ever produces a germ-cell, and the reason why children resemble parents and ancestors is because the germ-cells which give rise to individuals in successive generations are produced from the germ-cells of the previous generation. The line of descent or inheritance, therefore, is from germ-cell to germ-cell, and not from parents. Unless the reader makes himself absolutely familiar with the thought expressed in these facts he will never understand the science of Embryology.

Dr. Archdall Reid expresses this truth in the following words. “The somatic cells of the parent, therefore, as far as we know, contribute no living elements to the child; they merely provide temporary