LAW OF DEVELOPMENT KNOWN AS VON BAEE's LAW. 81 



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 



