The Recapitulation Theory in Biology 23 



11. The Laws of Acceleration and Retardation. 



A notable series of investigations in which recapitulation 

 figured largely was inaugurated by Alpheus Hyatt in Boston 

 in the year 1866. Hyatt's original study was made upon fossil 

 shells of certain mollusks, but his method has been used by his 

 pupils and followers with various groups of mollusks and other 

 invertebrates, and even with the vertebrates. This school of 

 investigators has derived so much assistance from recapitulation 

 that they are now to be regarded as its chief defenders. In- 

 deed, one member of the school has declared that "If the em- 

 bryologists had not forestalled them, the paleontologists would 

 have had to invent the theory." "This," observes Cumings, 

 "may be considered as a fair sample of the general attitude 

 of paleontologists of the Hyatt school. . ." w 



It was suggested by Hyatt that the conditions for making 

 comparisons between development and the "phylocycle" by 

 means of fossil shells are exceptionally favorable. The shell 

 and its parts are highly significant of the structure of the animal, 

 and display in a unique way the important phases of its growth 

 from the later embryonic stages to old age. ^ Moreover, the 

 procession through geological time is abundantly shown in 

 the successive geological formations not only for the adult forms 

 but for the corresponding immature stages in every youthful 

 phase as well. 31 It is therefore possible for students of these 

 shells to compare immature stages directly with their own adult 

 condition and with the immature and adult conditions of their 

 predecessors and descendants. These favorable circumstances 

 make the conclusions of this group of investigators of peculiar 

 value within the limits of their field of study. 



In making these comparisons Hyatt found certain relations 

 prevailing between the development of the individual and the 

 ancestral series, and summarized them by means of a restate- 

 ment of recapitulation, along with an induction original with him, 

 since known as the law of acceleration. . His own words for 

 these two generalizations are: 



10 For a valuable summaiy of the work of this school, with bibliographies, see an 

 article by this writer. Paleontology and the Recapitulation Theory, in Proceedings of 

 the Indiana Academy of Sciences, 1908-09 : pp. 305-34O. 



11 Phylogeny of an Acquired Characteristic. Proceedings American Philosophical 

 Society. 1893: p. 350. 



