! The PRiNCirLEs of Palaeontology. 175 



§ 3. EMBRYoaENio Method. 



Law of the parallelism of Ontogeny and Phylogeny. — The 

 second method appeals to researches still more delicate and in 

 which less advance has been made. Emhryogeny is a recent science, 

 whose progress must necessarily follow our knowledge of adult 

 forms. But already in many cases it has enabled us to elucidate 

 questions which Comparative Anatomy left unsatisfied. Even 

 palaeontologists have for some years past been earnestly seeking 

 for the results which this science furnishes. 



The importance of Embryogeny rests entirely in the appli- 

 cation of a law which has been the point of departure of most of 

 the researches lately made on the subject of the development of 

 organisms. Discovered by Kielmeyer and Geoff roy Saint- Hilaire, 

 formulated by Serres in regard to the human species, defined 

 more precisely and generalized by Haeckel, this law, verified and 

 restricted by later researches, is still- one of the most fecund 

 principles of the transformist doctrine. It consists in this, that 

 in a general manner, before arriving at the adult state, animals, 

 in the course of their developm'-nt, pass through the diverse 

 stages which marked the progress of the evolution of their 

 ancestors. In other words, according to the celebrated formula 

 of Haeckel, " Ontogeny is the abridged reproduction of Phy- 

 logeny." If this law is strictly true, it is evident, that no other 

 criterion is needed to reconstruct the entire genealogic tree of 

 the animal, since the diverse "forms constitute a gallery in minia- 

 ture of the portraits of their ancestors." 



In default of a direct verification, which, in the present case, is 

 evidently impossible, this law ma}^ be considered as proved by 

 numerous facts which admit of no other explanation. Many 

 animals reproduce in the course of their development the series 

 of the lower forms of the group. Such are, to confine ourselves to 

 classic examples, the anourous Batrachians, decapod Crustacea, 

 the Comatulas, etc. Examples of analogous facts are innumer- 

 able; we shall give in detail only a few selected from the 

 domain of Palaeontology. 



Embryogeny of fossil forms. — The earliest results in this order 

 of ideas are due to Wiirtemberger who, in 1873, applied these 

 principles to the Ammonites. 



In examining the forms of the group of Perisphinctes we see in 

 the oldest types the shell ornamented with ribs, two or three times 



