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."
Having in this brief preliminary consideration of the fundamental facts upon which the science of Embryology is based cleared the ground as far as possible, we may now summarise, in a few simple statements, the point at which we have arrived in order that we may proceed at once to the more detailed study of the actual development of the embryo itself.
We are in search of as clear a statement as possible of the origin of the many and varied characteristics which go to the formation of a human embryo, and hence to the making of an individual. The variation in these many characteristics accounts for the differences in individualities. No two individuals are exactly similar whatever be the standard by which we estimate them. This is true morally, ethically, and physically. In each of these spheres there are to be found good, bad, and indifferent individuals, but whichever they are it is quite obvious that the result has been brought about by the influence of all the factors of heredity and environment acting upon the capacities which were originally implanted in the germ-plasm. An individual is the resultant of the play upon one man's-worth of human material of all forces which have acted, or are acting, upon that kind and amount of material. Even though two children of the same parents be brought up under what are to all appearances identical circumstances, they differ from the very beginning from each other and their parents. This is true even of physical characteristics, and even more markedly in mental features. The fact is--and it is one which is not sufficiently recognised--that the formation of an individual from an embryo, the making of a man, is a biological problem fundamentally.