PLASTICITY AND PERMANENCY OF CHARACTERS. 299 



conditions is less interfered with. The species will therefore exist under 

 one or more permanent forms, according as portions of it have been iso- 

 lated at a more or less remote period." 



Permanency of Characters in Living Forms Co-ordinate with 

 Limitation in Distribution and Breeding. — From these quota- 

 tions it will be seen that in the conception of an organic spe- 

 cies the fundamental idea here emphasized is the reproduction 

 of numerous individuals possessing likeness in all their mor- 

 phological characters, except in those in which the offspring 

 of a single brood may differ when compared together. This 

 specific permanency involves absence of intermixing of the 

 separate species, if in the same locality, or local separation of 

 the species. In other words, co-ordinate with the likeness of 

 form there is assumed to be limitation in breeding and limita- 

 tion of local environment. This is the extent of the limita- 

 tion which the study of living forms reveals. 



Specific Variability Restricted with each Successive Generation 

 in Fossil Forms. — When we examine geological species we 

 find also a limitation in time of the repetition of like individ- 

 uals. When species are studied historically, the law appears 

 evident that the characters of specific value — those which 

 serve to distinguish one species from another, according to 

 the rules above formulated and generally practised — present 

 a greater degree of range of variability at an early stage in 

 the life-period of the genus than in the later stages of that 

 period. To express this law in terms of the history of organ- 

 isms, we say there are periods in the history of particular 

 lines or races of organisms, of unusual variability or plasticity 

 of some of the characters, and afterwards the history shows 

 relatively long periods in which the characters expressing 

 such plasticity are constant or present very slight divergence. 

 Further, in this second period of slow modification, or persist- 

 ence of form, the changes taking place in the phylogeny are 

 slight, but they increase in a particular direction steadily and 

 slowly with time. 



Illustrations of the Acquirement of Permanency of Charac- 

 ters. — In order to illustrate these laws the following actual 

 cases will be described in detail : the Spirifers at the base of 

 the Silurian, as an illustration of extrinsic evolution; Atrypa 



