176 THE EVOLUTION THEORY 



stage must always contain within it the primordia of the later stages 

 which have been added since the corresponding phylogenetic stage. 

 It is certain that the egg-cell or the sperm-cell of Man contains, 

 though in a form not recognizable by us, all the determinants of the 

 perfect human body, but this neither affects its nature as a cell nor its 

 particular form as ovum or spermatozoon. It is essentials that are 

 important in this comparison, not accessories. Neither can I agree 

 with Hensen's argument when he says that the ' recapitulation-idea ' 

 is erroneous, because the actual course of ontogeny is the ' best and 

 only possible one,' which, apart from previous history altogether, 

 must of necessity be followed. Certainly the actual course is the 

 best, and under the given circumstances the only possible one, but 

 that does not exclude recapitulation, on the contrary it implies it, for 

 ontogeny could at no time have arisen from a tabula rasa, but only 

 from what was historically existent. 



I do not propose to examine each of Haeckel's ancestral stages 

 in Man's pedigree, or to estimate the degree of probability with which 

 they may be deduced from the ontogeny ; but that Man's ancestry 

 does, in a general way, include such a series of phyletic stages may 

 be admitted, even if we grant that many of these stages are now 

 no longer represented in the ontogeny as stages of the developing organ- 

 ism as a whole, but only by stages of individual organs or group of 

 organs. Thus it may be disputed whether there is still a fish-stage in 

 human development, but it cannot be disputed that the rudiments of 

 ' gill-arches ' and ' gill-clefts,' which are peculiar to one stage of 

 human ontogeny, give us every ground for concluding that we 

 possessed fish-like ancestors. 



As we now know that the history of a given mode of embryo- 

 genesis has involved numerous time-displacements of the organ- 

 rudiments, we must attach all the more weight to the developmental 

 history of the individual parts and characters, in which the phylogeny 

 can often be read more clearly than in the stages of the organism 

 as a whole, and we can probabl}^ find out important laws in this way. 



As far back as 1873 Wiirtemberger investigated the fossil 

 ammonites with special reference to this point. He was concerned 

 even more at that time with finding proofs of the theory of descent 

 in general, and this was the first case in which any one succeeded 

 in demonstrating phyletic transformation-series of species, deposited 

 one above the other in a corresponding series of geological strata, 

 and connected by transition forms lying between these. In studying 

 this interesting material, of which many examples were at his disposal, 

 Wiirtemberger proved that the variations which had taken place 



