THE INFLUENCE OF PHYLOGENY ON ONTOGENY. 7 



of its species) have passed from the earliest periods of so- 

 called organic creation down to the present time. 



The causal nature of the relation which connects the 

 History of the Germ (Embryology, or Ontogeny) with that 

 of the tribe (Phylogeny) is dependent on the phenomena 

 of Heredity and Adaptation. When these are properly 

 understood, and their fundamental importance in deter- 

 mining the forms of organisms recognized, we may go 

 a step further, and say: Phylogenesis is the mechanical 

 cause of Ontogenesis. The Evolution of the Tribe, which 

 is dependent on the laws of Heredity and Adaptation, effects 

 all the events which take place in the course of the Evolution 

 of the Germ or Embryo. 



The chain of different animal forms which, according to 

 the Theory of Descent, constitutes the series of ancestors, or 

 chain of forefathers of every higher organism, and hence 

 also of man, always forms a connected whole. This un- 

 broken succession of forms may be represented by the letters 

 of the Alphabet A, B, C, D, E, etc., down to Z, in their 

 alphabetical order. In apparent contradiction to this, the 

 history of the individual evolution, or the Ontogeny of most 

 organisms show us only a fragment of this series of forms, so 

 that the interrupted chain of embryonic forms would be 

 represented by something like : A, B, F, H, I, K, L, etc. ; or, 

 in other cases, thus : B, D, H, L, M, N, etc. Several evolu- 

 tionary forms have, therefore, usually dropped out of t'he 

 originally unbroken chain of forms. In many cases also 

 (retaining the figure of the repeated alphabet) one or more 

 letters, representing ancestral forms, are replaced in the 

 corresponding places among the embryonic forms by equi- 

 valent letters of another alphabet. Thus, for example, in 



