"Thus," Spencer concludes, "of these four groups each furnished several arguments which point to the same conclusion; and the conclusion pointed to by the arguments of any one group, is that pointed to by the arguments of every other group. This coincidence of coincidences would give to the induction a very high degree of probability, even were it not enforced by deduction. But the conclusion deductively reached is in harmony with the inductive conclusion."
[9] See J. Arthur Thomson, _The Science of Life_ (1899), chapter xvi., "Evolution of Evolution Theory"; and _The Study of Animal Life_ (1892), chapter xviii., "The Evolution of Evolution Theories."
_Problems of Heredity--Physiological Units--A Digression--The Germ-Cells--Transmission of Acquired Characters--Inconceivability--A Priori Argument--Practical Conclusion_
Heredity is the relation of genetic continuity which links generation to generation. An inheritance is all that the organism is or has to start with on its life-journey in virtue of the hereditary relation to parents and ancestors. In all ordinary cases, the inheritance has its initial material basis in the egg-cell and the sperm-cell which unite in fertilisation at the beginning of a new life, and these two kinds of germ-cells, which bear the maternal and the paternal contributions, have their peculiar virtue of reproducing like from like, just because they are the unchanged or very slightly changed cell-descendants of the fertilised ova from which the parents arose. A bud or a cutting separated off from a living creature--tiger-lily or potato, polyp or worm--reproduces an entire organism like the parent, if the appropriate nurture-conditions are available; and it can do so because it is a fair sample of the parental organisation. Similarly a germ-cell or two germ-cells in conjunction can develop into a creature like the parent or parents, in virtue of being the condensed essence of the parental organisation. And the germ-cell is this because of its direct continuity through undifferentiating cell-divisions with the original germ-cell from which the parental body developed.
Even in ancient times men pondered over the resemblances and differences between children and their parents--for like only _tends_ to beget like--and wondered as to the nature of the bond which links generation to generation. But although the problems are old, the precise study of them is altogether modern. The first great step towards clearness was the formulation of the cell-theory by Schwann and Schleiden (1838-9), by Goodsir and Virchow, which made it clear that all but the simplest organisms are built up of cells or modifications of cells, and that the individual life usually begins as a fertilised egg-cell which proceeds by division and re-division, by differentiation and integration, to develop a more or less complex "body." It has become gradually clear that while the fertilised egg-cell gives rise to body-cells which become specialised, it also gives rise to unspecialised descendant-cells, which take no share in body-making, but become the germ-cells--the potential starting-points of another generation. A second great step was the accumulation of facts of inheritance showing that all sorts of qualities innate or inborn in the parents, essential and trivial, normal and abnormal, bodily and mental, may be transmitted to the offspring as part of the organic heritage. A third great step was implied in the acceptance which Darwin in particular won for the general idea of descent, for it is hardly too much to say that the scientific study of the problems of heredity began when it was recognised that heredity is a fundamental condition of evolution.