90 ROBERT TRACY JACKSOX ON 



aiDpeaniuce in the young. Senile stages are prophetic of types which may be found in 

 regressive series of the group to wliich the animal belongs, as shown by Hyatt in cephal- 

 opods and by Beecher in l>racliiopods. Senility is marked by the loss or reduction of 

 those characters wliich are fully evinced in the adult as specific and generic characters, 

 and Ijy the taking on of features of its own period of growth. 



To (luote Hyatt ('97), " The cycle of the ontogeny is, therefore, the individual expres- 

 sion and abbreviated recapitulation of the cycle that occurs in the phylogeny of the same 

 stock ; and, while the embryonic, nepionic, and neanic stages give us, in abbreviated shape, 

 the record of the epacme, the gerontic stages give, in a similar manner, the history of the 

 paracme." Professor Hyatt formulated the law of senile characters as follows : In the 

 old age, stages are found, which are similar to stages found !n the young, and are pro- 

 phetic of types to he found in degradational series of the group to which the animal 

 belongs. 



The above principles and their application, Hyatt's and Cope's principle of accelera- 

 tion of development, the well-known principles of parallelism, and the dynamic relations 

 of the organism to its environment of Cope, Hyatt, and Ryder, form the central working 

 principles of Prof. Alpheus Hyatt and his followers, and may be justly termed the Hyatl 

 school. The pi-inciple of localized stages in development as described in this paper is in 

 the direct line and the natural outcome of this method of work. 



Studying organisms in accordance with the above laws, one expects to find stages, in 

 a more or less marked degree, in the immature organism and also in the old age. In the 

 adult one expects to find the full species characters evinced, and, excepting as the adult 

 represents one step in a chain of organic series, stages are not looked for. 



From observations on animals and plants, evidence has been obtained which points to 

 the conclusion, that, in addition to stages in the young, and in the old age, stages may be 

 found in localized parts throughout the life of the organism. 



In organisms that grow by a serial repetition of parts, it is found that there is often 

 an ontogenesis of such parts, which is more or less closely parallel to the ontogenesis of 

 the organism as a whole. In the ontogeny of sucli localized parts in a mature individual, 

 we find stages in development during the growth of the said parts which repeat the' 

 characters seen in a similar part in the young individual. To state it briefly for the 

 moment, such localized stages have been observed in the leaves of plants, in branches or 

 suckers of plants, in the budding of some lower animals, as Hydra and Galaxea, in the 

 plates of crinoids and Echini, in external ornamentations in mollusks, and in the septa of 

 cephalopods. 



Localized stages in development must be clearly differentiated from stages in develop- 

 ment of the organism as a whole. They are features seen in localized parts throughout 



