The fertilized egg gives origin, as we have seen, to the multitude of cells which build up the body of one of the higher animals. There are, on the one hand, muscle-cells, gland-cells, nerve-cells, and other constituents of the various tissues; and there are, on the other hand, the reproductive cells--ova or sperms, as the case may be. Now, every cell in the developed animal is a direct descendant of the fertilized egg. But of all the varied host only the reproductive cells take any direct share in the continuity of the race. Hereditary transmission is therefore restricted to the germinal substance of these reproductive cells. Trace the ancestry of any cell in the adult body, say a nerve-cell, and you reach the fertilized ovum. Trace back the ancestral line yet further, and you follow a long sequence of reproductive cells, or, at least, of cells which have undergone but little differentiation; but never again will you find, in the course of a genealogy of bewildering length, a nerve-cell. Such a tissue-element is a descendant, but cannot become an ancestor; it dies without direct heirs. It is universally admitted that the bodily structures are subject to what is termed _modification_ under the stress of environing circumstances. The muscles may acquire unusual strength by use and exercise; the nerve-centres may learn certain tricks of behaviour in the course of individual life; and other structures may be similarly _accommodated_ to the conditions which affect them. To such modifications of structure or function in the organs or parts the term _acquired_ is primarily applied. The tissues have thus a certain amount of organic plasticity, through which they are adjusted to a range of circumstances varying in extent. They are able to acquire new modes of behaviour. But the cells of which they are composed are off the line of racial descent. They leave no direct heirs. When the body dies the modifications of behaviour acquired by its parts perish with it. Only if in some way they exercise what we may term a homœopathic influence on the germinal substance can the accommodation they have learnt be transmitted in inheritance. By a _homœopathic_ influence is here meant one that is of such a nature as to communicate to the germinal substance, the seeds of similar changes of structure or function. And of the occurrence of any such homœopathic influence there is no convincing evidence.