PLASTICITY AND PERMANENCY OF CHARACTERS. 297 



increment is the great fact; the checking and limiting of it is 

 secondary. The search has been for some cause of the varia- 

 tion; it is more probable that mutability is the normal law of 

 organic action, and that permanency is the acquired law. 



It is more probable that the use and tested adaptability 

 of a variable part is the cause of checking the variability and 

 of the transmission of the character with less or no variation, 

 than that the variation is increased by this process. Adopt- 

 ing mutability as a fundamental law of all organic activity, 

 and the construction of a theory of evolution becomes a simple 

 matter. We have in that case to account for the acquirement 

 of permanency of characters. This is found in the principle 

 of ordinary generation, the instituting of habit, and the more 

 and longer the species breed together the closer and more 

 fixed will the characters become. ^ 



Early Plasticity Succeeded by Permanency expressed in Geo- 

 logical History. — Examination of the history of geological spe- 

 cies suggests the truth of this hypothesis, for it is observed 

 that many species, which by their abundance and good preser- 

 vation in fossil state give us sufficient evidence in the case, 

 exhibit greater plasticity in their characters at the early stage 

 than in later stages of their history. A minute tracing of 

 lines of succession of species shows greater plasticity at the 

 beginning of the series than later, and this is expressed in the 

 systematic description and tabulation of the facts by an in- 

 crease in the number of the species. 



In order to illustrate this law a special consideration will 

 now be given to the facts regarding the laws of specific his- 

 tory as observed by the paleontologist. 



Pritchard's Definition in which the Constancy of Transmission 

 of Same Peculiarity is made the Criterion of Species. — Thus far 

 wc have been considering generic characters — that is, those 

 characters which are constant for one or more species. The 

 next question to consider is. What are the laws exhibited 

 in the history of specific characters? There are various defi- 

 nitions of species which are more or less theoretical ; but 

 whatever our theory about the definition, the fact remains 

 that all naturalists do recognize within slight limits of difference 

 the reality of groups of organisms called by the name species. 



