CHAPTER VIII
THE BEGINNINGS OF THINGS (continued)
We have now considered, as far as is compatible with the character of a work of this kind, the beginning and development of the embryo taken as a whole, and for the remaining part of our study of this subject we may devote our attention to the beginnings of some of the more important organs and functions in the new individual. It will be impossible to deal in detail with all the important parts which ultimately constitute the new personality, but a selection may be made which will give some general idea of how great results spring from very small beginnings. What will be said here it may be hoped will be just sufficient to stimulate the interest of those to whom the subject appeals, and who may then turn their attention to some of the larger works which go into greater detail in this subject, a list of which will be found in the bibliography at the end of this volume.
It must be remembered that quite a large number of the characteristics that we usually associate with a normal human being only come into existence, or at any rate only become obvious, at some period longer or shorter after birth. True, these characteristics depend for their ultimate appearance upon the development of the corresponding structures and organs in the growing embryo, but in the case of some of these, those organs are not fully developed in embryonic life, and the manifestation of the functions associated with them may be delayed perhaps for years. This is notably the case, for example, with the reproductive organs which, though developed during the life of the embryo, remain functionless until the period of adolescence. The development of the human mind and intellect too, although depending, of course, upon the embryonic growth of the brain and the nervous system generally, is a matter of time and the environment subsequent to birth. It should be realised, however, in this connection, that the mind of the new individual, and all that is involved in that term, dates back ultimately, as regards its possibilities, to the moment at which the two germ-cells from the male and female respectively united in fertilisation. The adult mind develops from the mind of the infant. The infant mind appears as the result of the possibilities and the tendencies which were inherent in the germ-cells from which not merely the brain but the whole embryo sprang; in other words, all that a single human mind connotes results from the possibilities in a single cell. Such a thought is a startling one indeed, and at first sight appears, perhaps, somewhat incredible. But a moment's careful attention to the problem will show at once that it is in reality no more wonderful than the fact that this single cell produces all the millions of other cells which in due time give rise to the skin, bone, nerve, blood, and so forth, which make up the entire body of the embryo. The human mind, therefore—and indeed the human soul, if that term be used in any intelligible sense—takes its origin in the products of the multiplications of germ-cells acted upon by their subsequent surroundings.[1]
With this passing reference to the fact that some important parts of an individual only grow to their full manifestations after embryonic life, we may pass to the consideration of the development of some of the more interesting parts of the embryo itself.
Amongst the most striking, and certainly the most interesting, of the various parts of the developing embryo, those which go to form the special senses are prominent. They are interesting not merely from their actual mode of growth, but especially also in connection with their evolutionary history. The study of how they have come into their present state in the higher animals leads us back to very small beginnings—indeed, to the time when there was no such thing as special sense organs for sight, hearing, smell, and so forth, but where the organism had what may be termed a diffused tactual sense over and throughout the entire body. In the course of time this diffused general sense became specialised, no better example of which could be quoted than that of the sense of sight, which was referred to, as many of our readers will doubtless remember, in Tyndall's famous Belfast address. He was referring to Herbert Spencer's theory of the manner in which vision was evolved. He pointed out that, as above noted, in the lowest organisms sensation is a general thing diffused throughout the body, a kind of general tactual sense. As the result of environment, and gradual adaptation to external influences, certain parts of the general surface of the organism became more responsive to these external stimuli than other parts. These areas, being those points at which sensation was most acutely felt, were nothing more or less than primitive sense organs. Thus in the progress of evolution the stimulus of the eye gradually became most pronounced in certain cells which contain pigment, these cells being more responsive to the light stimulus than the rest of the body. That was the beginning of an eye; a group of cells more receptive, more easily influenced by light, than any other cells. In a slightly higher stage of evolution we find a special overgrowth of the skin which covers over the area in which these pigmented cells lie, obviously a protective measure on behalf of the specialised cells referred to. Then, still later, a lens is added, and the whole organ becomes more and more adapted to the necessities of the case, until it reaches the extraordinary perfection that is seen in the eye of such a bird as an eagle. On the same general principle, the other special senses also took their origin from this general diffused tactual sense, certain cells becoming specially adapted for receiving the stimulus of sound, others for taste, others for smell, and so forth.
[1] The detailed study of this part of the subject is dealt with in the writer's work, The Greatest Life (Duckworth, London), to which readers who are interested in this phase of the subject are referred.