LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 4] 
organism reproducing the variations inherited from all its an- 
cestors at successive stages in its individual ontogeny” 
(‘Comp. Emb.,’ vol. i, p. 3). 
“These two principles, namely, that slight variations gene- 
rally appear at a not very early period of life, and are inherited 
at a corresponding not early period, explain, as I believe, all 
the above specified leading facts in embryology.” (Darwin, 
‘Origin,’ p. 392, ed. vi.) 
But this explanation, though good as far as it goes, is not 
entirely satisfactory, because it fails to explain (without further 
qualifications) the majority of cases (animal and plant buds, 
embryonic development of seeds) in which ontogeny presents 
no ancestral traces; it is at variance with the fact that in 
many cases variations which affect the adult have affected the 
whole of embryonic development (see below) ; and it does not 
enable us to understand why some organs, e.g. gill slits, have 
been retained in embryogeny, whereas other organs which have 
much more recently disappeared, e.g. teeth of birds, fore-limbs 
of snakes, have been entirely lost. It assumes that the repe- 
tition of ancestral characters in embryogeny is the intelligible 
rule; and that their omission is the exception which requires 
explanation whenever it occurs. This assumption is not 
warranted by the fact above indicated that in the vast majority 
of ontogenies there are no phylogenetic traces, nor by the 
consideration that a number of important organs, such as 
teeth and hand-claws in birds, limbs in snakes, gill-clefts in 
fishes, have recently disappeared without leaving a trace in 
ontogeny. 
In fact the balance of evidence appears to me to point most 
clearly to the fact that the tendency in embryonic development 
is to directness and abbreviation and to the omission of an- 
cestral stages of structure, and that variations do not merely 
affect the not-early period of life where they are of immediate 
functional importance to the animal, but, on the contrary, that 
they are inherent in the germ and affect more or less pro- 
foundly the whole of development. 
I am well aware that in holding this opinion I am running 
