46 ORGANIC EVOLUTION — PHYSICAL 



the cell-descendants of conjii seated infusorians, the cell- 

 descendants of germ cells, (1) instead of separating 

 remain adherent, and (2) the mass thvis formed, as cell- 

 proliferation proceeds, takes definite shapes, at lirst 

 resembling those of very low multicelhdar 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 de- 

 rived ; and (3) each successive generation of cells shows 

 greater and irreater degrees of differentiation and 

 specialization, till such highly differentiated and special- 

 ized cells as skin, nerve, gland, blood, &c. as are present 

 in the fully developed organism appear. In oilier 

 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 aftbrds 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 off- 

 spring 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 recajiitulates tlic traits of the parent, 

 the parent the traits of the grandparent, the grand- 

 parent the traits of the great-grandparent, and so on, it 

 is evident that the son, the last of the race, must re- 

 capitulate 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 



