The same truth applies to the cells which make up the body of the embryo and the individual, as well as to the germ-cells. The body-cells, those which make up bone, and muscle, and gland, and so forth, are constantly exposed to all sorts of influences which must tend to damage them so far as it is possible for the cells to be damaged and still live. These body-cells are sometimes starved, sometimes poisoned with alcohol and drugs, frozen by extremes of temperature, over-worked by too much physical strain, and so on, and if it were possible for such external influences to change the type of cells of their offspring we should expect to see it here. But it does not occur. The internal hereditary tendencies of these cells are so strong, and so intimately bound up with the life of the cells themselves, that when they divide and produce others these others are precisely similar to the parent cells, in spite of all the unfavourable environment in which they have been. Slight variations do, of course, occur, but these are chiefly of a germinal or spontaneous nature, and not due to the environment. This thought gives us some vague and imperfect idea of how immensely complex the constitution of germ-plasm must be. This germ-plasm is very often subjected to all sorts of unfavourable conditions, especially those of alcohol and toxins, and such conditions have been acting upon it more or less for an immense number of generations, and yet the resistance to modification at the hands of these internal factors is so great that all the processes which follow upon the fertilisation of the ovum, all the thousand complications which thereafter ensue in the building up of the young embryo are hardly ever interfered with. When they are markedly interfered with such interference generally involves the death of the embryo. The conclusion arrived at on this subject by Dr. Archdall Reid, after a very careful and extensive inquiry into all the evidence from many points of view, is stated by him as follows: "Though variations may result from the direct action of the environment, such variations are, in effect, always injuries, and are of rare occurrence in individuals who survive and have offspring. Adaptation (_i.e._ evolution) depends almost exclusively on spontaneous variations. These do not imply damage to the germ-plasm, but are products of its vital activity. Occurring in vast abundance all round the specific and parental means, they supply the sole material for Natural Selection." "We conceive the germ-plasm, then, as living and active, closely adjusted to its environment, growing, dividing, varying, capable of being destroyed and injured, but resisting death and injury, and within limits capable of repairing damage and returning to its original state--as behaving exactly as a living individual does." CHAPTER IV THE MAKING OF A MAN