THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 253 



of the term and the exact determination of any concrete 

 species are difficult to accompHsh. 



The examination has revealed the fact that the funda- 

 mental difference in opinion regarding species turns upon the 

 belief as to the mutability or iuumitability of species. 



The idea that species are mutable is intimately associated 

 with the inquiry, What is the "origin of species"? In at- 

 tempting to answer this question the deeper ones arise, i.e., 

 What is evolved in evolution? and What is mutable? 



The answer was that in any individual case all that is 

 evolved is to be found in the variation exhibited in those 

 characters by which it departs from the exact imitation of the 

 characters of its ancestors, and that evolution consists in the 

 acquirement of characters not possessed by the ancestors. 



We examined the classifications of the Animal Kingdom 

 particularly, and we found that, looked at analytically as 

 composed of avast number of different structures, or synthet- 

 ically as a multitude of related organisms variously differen- 

 tiated, and differentiated to various degrees along a few 

 general lines of evolution, the Animal Kingdom is divisible 

 into a number of definite groups, marked by definite organi- 

 zation, all the grander features of which were outlined in the 

 Cambrian age, and the large majority of all the differentia- 

 tions of even ordinal rank had been accomplished in the first 

 quarter of the recorded history of organisms. 



It is evident, therefore, that we must read the law of 

 evolutional history in terms of the genera and species as they 

 are distributed in families or in orders. 



Generic and Specific Evolution Illustrated by the Brachiopoda. 

 — In order to study the successive appearance of species and 

 genera, it will be necessary to turn from the more general 

 characters to the minuter marks distinguishing species from 

 species, or at least genera from genera. For this purpose no 

 better group of organisms can be selected than the Brachiop- 

 oda. In presenting the results of this analysis the paleon- 

 tologist will miss that elaboration of the facts which would 

 make the discussion of most practical use to him. The brief 

 limits of this introductory treatise do not admit of this; and if 

 the presentation of the facts shall stimulate some such readers 



