PARALLELISM. 191 



therefore parallel with the forms evolved in the para- 

 plastic or retrogressive stage of evolution of the phy- 

 lum. In other words, the morphic modifications which 

 may occur as permanent, specific, and generic charac- 

 ters in the adults of retrogressive descendants of any 

 progressive individuals may be predicted from the 

 study of the similar changes that take place in the 

 senile stages of the progressive individuals. As it has 

 been stated by the writer on several occasions, the 

 embryonic, nepionic, and later stages of development 

 up to the adult repeat with greater or less clearness in 

 proportion to their removal in time and organization 

 from the point of the origin of the genetic group to 

 which they belong the permanent characteristic of 

 their ancestors ; the adult gives the existing essential 

 differentials acquired by its own species, genus, and 

 group, being the index according to the time of its oc- 

 currence of the progression or retrogression of its 

 group; the old, in its invariably retrogressive course, 

 indicates the path that must be followed by degraded 

 series after the acme of the group to which the indi- 

 vidual belongs has been reached. This, of course, is 

 a generalized statement of the correlations of the on- 

 togenic cycle and the phylocycle when they occur as 

 in the Ammonitinae, but it will be found eventually 

 that this law is true of all animals to some degree. It 

 is obvious from all past experience that every law of 

 correlation of structures cannot be true in any one 

 group without being found more or less in all organ- 

 isms. I have therefore ventured upon the basis of 

 this and Beecher's, Clarke's, and Schuchert's re- 

 searches among Brachiopoda, corals, and trilobites. 

 Dr. Jackson's among pelecypods, and after the con- 

 firmations by the independent researches of \^'iirten- 



