PAL EONLOGLNY AND -PHVEOGENY 509 
Hyatt has given a more exact and comprehensive definition of the 
law of acceleration or fachygenesis: ‘‘ All modifications and varia- 
tions 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 sub-stages of develop- 
ment 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.” * Since Hyatt’s first paper 
the law has been rediscovered and renamed by Haeckel,3 ‘das 
biogenetische Grundgesetz”’ and by Wirtenberger.* But these 
naturalists, instead of adding anything to Hyatt’s definition, 
have failed to reach its clearness and simplicity. The only real 
addition that has been made is Cope’s$ idea of retardation, by 
which is explained the separation in the ontogeny of the descend- 
ant of characters that occurred simultaneously in the ancestor. 
Cope says: ‘The acceleration in the assumption of a character, 
progressing more rapidly than the same in another character, 
must soon produce, in a type whose stages were once the exact 
parallel of a permanent lower form, the condition of znexact 
paralleism. As all the more comprehensive groups present this 
relation to each other, we are compelled to believe that accelera- 
tion has been the principle of their successive evolution during 
the long ages of geologic time. Each type has, however, its 
day of supremacy and perfection of organism, and a retrogres- 
sion in these respects has succeeded. This has no doubt followed 
tA. Hyatt, Smithsonian Contributions to Knowledge, No. 673, “Genesis of the 
Arietidz,” Preface, p. ix. 
2 Proc. Am. Phil. Soc., Vol. XXXII, No. 143, A. Hyatt, “Phylogeny of an 
Acquired Characteristic,” p. 405. 
3“ Morphologie der Organismen,” Vol. II; and “ Anthropogenie,” 1874. 
4 Ausland, 1873, and “ Studien iiber die Stammesgeschichte der Ammoniten,” 1880. 
5 Origin of the Fittest. 
