History of the Theory of Heredity. 31 



to the two-lciyered gastriila : from this to the worm-like 

 and skulless germ, and from this to hiter stages which 

 repeat, essentially, the organization of fish, amphibian, 

 reptile, until at last we have a perfect bird. On the 

 contrary, we unravel from this history the correspond- 

 ing series of ancestral forms, which have led up through 

 the amoeba, the gastrsea, the worms, the acrania, the 

 fishes, the amphibia and the reptiles to the bird. 



*' The series of changes in the hen's Qgg gives us an 

 outline sketch of the series of ancestors. TIds ancestral 

 or pliylogenetic significance of tlie plienomena of ontogeny 

 or individual development is up to the present time the 

 only explanation of tlie latter^''' ('' Gesammelte Populare 

 Vortriige," IE., p. 103.) *' Any one who accepts the 

 law that individual development is a recapitulation of 

 the evolution of the species iv ill find it simply natural 

 that the microcosm of the ontogenetic cell-tree should 

 be the diminutive, and in part distorted, reflection of 

 the macrocosm of the phylogenetic genealogical tree of 

 the species." {'' Gesammelte Populiire Vortriige," II., 

 p. 68.) 



No one can set too high a value upon the scientific 

 law here expressed— that individual development is a re- 

 capitulation of the history of the evolution of the species. 

 It must be regarded as one of the greatest generaliza- 

 tions of modern science, but I do not think it is possible 

 to agree with Ilaeckel that with its discovery the mystery 

 of individual development has clearly revealed its deep 

 significance, and no longer faces us as a riddle. 



It may be true that it is "simply natural" that the 

 ^^g of a horse should recapitulate the ancestral history 

 of horses, and the Qgg of a bird the ancestral history 

 of birds, but the statement that this is the case is in no 

 sense an explanation of heredity. For that matter it is 



