4 1 6 JAMES PERRIN SMITH 



And this was exactly the method used by Louis Agassiz, 

 who first applied the law of acceleration of development to the 

 study of systematic zoology, although it never had much influ- 

 ence on biologic investigation until the palentologic studies of 

 Hyatt in the invertebrates and Cope in the vertebrates placed 

 the law on a sound basis. It was reserved for Alpheus Hyatt 

 to formulate the law and to strengthen theory with practical 

 examples based on study of cephalopods. In his later papers 

 Professor Hyatt 1 has given a more exact and comprehensive 

 definition of the law of acceleration or tachy gene sis : "All modi- 

 fications 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 characteristics of later origin." A still 

 more definite statement by the same author is the following: 

 "The substages 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." 2 



To insure trustworthy results in verifying this law, the inves- 

 tigator must have groups in which the larvae are primary and 

 reproduce ancestral characters ; in which the living and the 

 fossil are classified on the same basis ; of which we have pre- 

 served 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 especially represented among the 

 Coelejiterata, the .Echinodermata, the Brachiopoda, and the Pelecy- 

 poda and Cephalopoda among the mollusks. 



Unequal acceleration. — Now, when the morphologist has set- 

 tled the fact that primary larval stages do actually reproduce, 

 more or less vaguely, characters that existed in the adult fore- 

 fathers of the generation he is at work on, his troubles are even 



1 Genesis of the Arietidae, p. 9. 



2 Philogeny of an Acquired Characteristic, p. 405. 



