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We have no longer any right to believe in the old oft-refuted hypothesis which assumes that each individual organism produces germ-cells afresh again and again and transmits all its powers developed and acquired by the parents; but, on the contrary, we have come to know to-day that parents are nothing but mere channels through which these germ-plasms or germ-cells manifest their peculiar tendencies and powers which existed in them from the very beginning.
It is even possible that the effects of these influences may be more specialized; that is to say, they may act only upon certain parts of the germ-cells.
When we think of a man or woman, we think of an individual only one of whose innumerable activities reproduction is carried on by germ-cells, and this one only at the very beginning of the life of a new individual. Human societies, needless to remark, are not organized by germplasms, but by brains and hands composed of body cells.
For many decades past theory has perceived that, in the germ-cells whence we and the higher animals and plants are developed, there must exist somewhere intermediate between the chemical molecule and the vital unit, the cell itself units which Herbert Spencer, the first and greatest of their students, called physiological or constitutional units.
But the very constitution of the germ-plasm and its relation to the body absolutely forbids the transmission of acquired somatic characteristics and of the special effects of use and disuse. Muscular activity promotes general health, and might thus conduce to better-nourished germ-cells and to more vigorous and therefore athletic descendants.
The process of cell-division, whereby the germ-cells or gametes are made, is called gameto-genesis. Somewhere in its course there occurs the capital fact discovered by Mendel and called by him segregation. A cell divides into two which are the final gametes. One of these will definitely contain the Mendelian factor, and the other will be as definitely without it.
Here there must be scattered through the leaves of the plant small portions of germ-plasm, which generally remain dormant, and only under special conditions increase and give rise to germ-cells. A large part of the germ-plasm of the fertilized egg is used to give rise to the somatoplasm composing the different systems of the embryo and adult.
How far this can be shown to be probable is, of course, another matter, but I am not immediately concerned with this; all I am concerned with now is to show that the germ-cells not unfrequently become permanently affected by events that have made a profound impression upon the somatic cells, in so far that they transmit an obvious reminiscence of the impression to the embryos which they go subsequently towards forming.
I am also compelled to admit it as conceivable that organisms may exert a modifying influence upon their germ-cells, and even that such a process is to a certain extent inevitable. The nutrition and growth of the individual must exercise some influence upon its germ-cells . . . "
It does not flow circuitously from egg to adult and then to new germ-cells, but it is direct and continuous, and apparently it cannot pick up any of the body-changes of an acquired nature. Now we see why individual acquisitions are not transmitted.
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