THE THEORY OF RECAPITULATION. 81 
The Theory o£ Recapitulation : A Critical Re-statement o£ the Biogenetic 
Law. By Walteb Gabstang, M.A., D.Sc.(Oxon), Professor of 
Zoology in the University of Leeds. (Communicated by Prof. W. 
Bateson, F.R.S., F.L.S.) 
(With a Text-figure.) 
[Read 2iid June, 1921.] 
MouE than half a century has passed since Haeckel propounded his " funda- 
mental Biogenetic Law" (1866). It played a great part in the campaign 
for the recognition of Evolution, has inspired and still inspires much good 
work in Palseontology, but, as a working hypothesis in Embryology, is 
admitted to have evoked little but controversy and confusion. This history 
alone renders it probable that the law is a mixture of sound and questionable 
elements, but the two have never been satisfactorily disentangled. The late 
Dr. C. H. Hurst (1893), Adam Sedgwick (1894 & 1909 (a)), and GeofErey 
Smith (1911), Oscar Hertwig (1898 & 1896), and Morgan (1908), among 
others *, have criticised particular aspects of it, but no one has presented a 
complete theoretical scheme capable of replacing Haeckel's as an explan- 
ation of the relations between ontogeny and phylogeny. Lately MacBride 
(1914 & 1917), from the embryological side, and Bather (1920), from the 
standpoint of palaeontology, have revived the full Haeckelian doctrine ; and 
the former has even considerably extended it, though neither, so far as I can 
see, has refuted, or even appreciated, the force of the criticisms made by 
their predecessors. As it is not to the credit of science that Zoology should 
harbour a " law " which, like a creed, may be accepted or rejected at 
pleasure, and as I believe the basis of this law is demonstrably unsound, 
I venture to make a renewed attempt to define the points at issue. The most 
satisfactory way of doing this appears to be to re-state, in accord with 
modern knowledge, the theoretical relations of ontogeny to phylogeny, and 
then to subject the alternative theories to verification by test-cases. As the 
old law was essentially morphological, I exclude from present consideration 
all bionomical and setiological questions not directly involved. 
1. The two aspects of Haeckel's doctrine — the statement of fact and the 
theory of causation — were summed up by himself in the phrases : " Onto- 
genesis is the recapitulation of Phylogenesis'^ and "Phylogenesis is the 
mechanical cause of Ontogenesis." In these now familiar terms the new 
* Bateson's criticism of the law of von Baer, though not specially referring to Haeckel's- 
modification of it, should be included here (1894, pp. 8-10). 
LINN. JOUKN. — ZOOLOGY, VOL. XXXV. 6 
