the cell-descendants of conjugated infusorians, the cell-descendants of germ cells, (1) instead of separating remain adherent, and (2) the mass thus formed, as cell-proliferation proceeds, takes definite shapes, at first resembling those of very low multicellular organisms, then with more or less indistinctness those of higher organisms, and lastly that of the parent organisms, from which the pair of germ cells which conjugated, and from the union of which the other cells resulted, was derived; and (3) each successive generation of cells shows greater and greater degrees of differentiation and specialization, till such highly differentiated and specialized cells as skin, nerve, gland, blood, &c. as are present in the fully developed organism appear. In other words, the development of the individual is a short, rapid, blurred recapitulation of the evolution of the species; which, if there is any truth in the theory of evolution, is exactly what was to be expected, and which therefore affords convincing proof of its truth.
We have seen that evolution depends on three factors: (1) that offspring in general inherit (i.e. recapitulate) the traits of their parents; (2) that offspring vary somewhat from their parents; (3) that there is invariably a struggle for existence, during which natural selection causes evolution by preserving favourable and eliminating unfavourable variations. Now if the son recapitulates the traits of the parent, the parent the traits of the grandparent, the grandparent the traits of the great-grandparent, and so on, it is evident that the son, the last of the race, must recapitulate the traits of each ancestor up to the remotest, or at least up to the unicellular organism, which for convenience of language we may call the first ancestor. In other words, the last descendant recapitulates the traits of the first ancestor, plus the traits, in their order, of all subsequent ancestors, beginning with the traits
