232 



FOOT-NOTES TO EVOLUTION. 



biogenetic law, in his formula, " Gleichung zwischen der 

 Entwicklung des Embryo und der Thierreihe," com- 

 parison of development of the embryo with the race of ani- 

 mals. But Louis Agassiz, although not the discoverer, 

 was undoubtedly the first to use the law as an aid in 

 the systematic study of biology. While he regarded the 

 various genera, not as ancestors and descendants, but 

 as progressive steps in creation, still he saw the analogy 

 between the stages of growth of the individual and 

 these progressive steps. It was reserved for Alpheus 

 Hyatt to formulate the law and to strengthen theory 

 with practical examples based on the study of cephalo- 

 pods.* In his later papers Professor Hyatt has given a 

 more exact and comprehensive definition of the law 

 of acceleration or tachygenesis : " 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 charac- 

 teristics of later origin." f A still more definite state- 

 ment by the same author 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." J Since Hyatt's first paper the 



* A. Hyatt. Mem. Boston Soc. Nat. Hist., vol. i, 1866-67; 

 and Proc. Boston Soc. Nat. Hist., vol. i, 1866, Parallelisms of In- 

 dividual and Order among the Tetrabranchiate MoUusks. 



f A. Hyatt. Smithsonian Contribution to Knowledge, No. 

 673, Genesis of the Arietidse, Preface, p. ix. 



X Proc. Am. Phil. Soc, vol. xxxii, No. 143, A. Hyatt, Phy- 

 logeny of an Acquired Characteristic, p. 405. 



