l82 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Introduction. 



The interest of the paleontologist in embryology, and in 

 ontogeny in general, lies wholly in the desire to know the 

 origin and relationships of biologic groups. A scientific inter- 

 pretation of ontogenic data in terms of phylogeny depends 

 on the extent of preservation of the ancestral record in 

 individual development. The broad statement has often 

 been made that each animal gives in its own development an 

 epitome of the history of its race. Because of the law of 

 heredity, this statement would be true, and the record would 

 be complete, if nothing had interfered with the normal 

 course of things. But, in reality, so many secondary ele- 

 ments are introduced in development, that authorities are 

 very much divided as to the value of ontogenic stages as 

 records of race history.^ 



There can be no doubt that students of postembryonic 

 stages have been inclined to claim too much for the law of 

 tachygenesis, while, on the other hand, students of embry- 

 ology have been inclined to discredit it almost entirely, and 

 to lay little stress on ontogenic stages as a recapitulation of 

 phylogeny. The reason for this disagreement is not far to 

 seek; it lies in the field and in the methods of research of 

 the two groups of morphologists. 



Types of Development. — Leaving out of consideration the 

 Protozoa, which come into being with the essential charac- 

 teristics of the adults, there are, in the Metazoa, two types 

 of development: (i) the foetal type, in which develop- 

 ment takes place in the ^^^, or in the body of the parent, 

 and the young animal comes into the world in form closely 

 resembling the adult; (2) the la?'val type, in which the 

 young animal comes out at an earlier stage of development, 

 and reaches maturity only after considerable metamorphosis. 



Secondary elements will be introduced in either type of 

 development, and those variations that are favorable to the 



1 In the preparation of this Introduction the writer has drawn largely on Balfour's 

 " Treatise on Comparative Kmbryology," and on Lang's " Comparative Anatomy." 



