254 THE TRANSFORMATIONS OP THE ANIMAL WORLD 



transformed into an instrument of research, and 

 into an obligatory criterion of all studies on the 

 origin and the pedigree of living and fossil beings. 



It is indisputable, if we only consider the most 

 general features, that the history of the develop- 

 ment of an individual is a kind of rapid recapitula- 

 tion of the slow phases of the evolution of the species 

 and of the branch. This recapitulation is, more- 

 over, very often shortened and simplified, especially 

 in the most differentiated groups, by the fact that 

 the embryo passes through certain stages very 

 rapidly, or even suppresses them altogether. This 

 phenomenon has received the name of embryogenic 

 acceleration or T achy genesis. The embryology of 

 living animals has furnished numerous proofs in 

 support of these laws. Without dwelling on the 

 point, I will confine myself to recalling the case of 

 the Cirrhipedes, so different from the Crustacea in 

 the adult stage that they were taken for Molluscs, 

 and with larvae which develop themselves after 

 the Nauplius type, like that of the Crustacean 

 Ostracods, Phyllopods, and Copepods. I will also 

 remind the reader that the embryos of all classes 

 of the Vertebrates resemble each other in the first 

 stages of their development to such a degree that 

 it is difficult to distinguish one from the other, and 

 that they only acquire little by little the charac- 

 teristic traits of each group. 



Has Palaeontology completely confirmed the 

 conclusions thus drawn from the embryology of 

 existing beings ? We may approach this important 

 question by two different methods. 



The first method, which is the oldest and the 



