Geol.-Vol. I.] SMITH— PLACENTICEK AS. 1 85 



And this was exactly the method used by Louis Agassiz, 

 who first appHed the law of acceleration of development to 

 the study of systematic zoology, although it never had much 

 influence on biologic investigation until the paleontologic 

 studies of Hyatt (1872) in the invertebrates, and Cope in 

 the vertebrates placed the law on a sound basis. It was 

 reserved for Alpheus Hyatt (1866) to formulate the law, and 

 to strengthen theory with practical examples based on study 

 of Cephalopoda. In his later papers Professor Hyatt (1889, 

 preface, p. ix) has given a more exact and comprehensive 

 definition of the law of acceleration or tacky genesis: "All 

 modifications and variations in progressive series tend to 

 appear first in the adolescent or adult stages of growth, and 

 then to be inherited in successive descendants at earlier and 

 earlier stages according to the law of acceleration, until 

 they either become embryonic, or are crowded out of the 

 organization, and replaced in the development by character- 

 istics of later origin." A still more definite statement by 

 the same author (Hyatt, 1894) is the following: "The sub- 

 stages of development in ontogeny are the bearers of distal 

 ancestral characters in inverse proportion and of proximal 

 ancestral characters in direct proportion to their removal in 

 time and position from the protoconch or last embryonic 

 stage." 



To insure trustworthy results in verifying this law, the 

 investigator must have groups in which the larvae are pri- 

 mary and reproduce ancestral characters; in which the 

 living and the fossil are classified on the same basis; of 

 which we have preserved a nearly complete geologic record; 

 and of which material is available for the study of fossil 

 ontogeny as a check on the living. Such groups are espe- 

 cially represented among the Ccelenterata, the Echinoder- 

 niaia, the B?'achiopoda, and the Pelecypoda and Cephalopoda 

 among the molluscs. 



Unequal Acceleration. — Now, when the morphologist has 

 settled the fact that primary larval stages do actually repro- 

 duce, more or less vaguely, characters that existed in the 

 adult forefathers of the generation he is at work on, his 

 troubles are even then not yet ended; for the characters do 



