The Principles of Palaeontology. 139 



which are frequently most unquestionable; furthermore, it is 

 well understood that variations of detail are of considerable 

 importance since they are characteristic of different horizons. 

 Thus the palaeontologic works of that epoch were carried on 

 with a precision and care, as they continue to be in their entirety 

 at the present day, and are a point of departure for effectual 

 researches in each group. 



The progress of Palaeontology during the first half of this cen- 

 tury is due to the efforts of scientists who prosecuted their re- 

 searches in two different ways. On the one hand the theoretic 

 discussions among zoologists which have made so great noise in 

 the world, have for their object the problem of the species, and 

 have given rise to the doctrine of Evolution. On this account 

 they are of direct interest to the palaeontologist. On the other 

 hand, the geologists are engaged in exploring with the greatest care 

 the deposits of all the countries of Western Europe, and describe 

 minutely the fossils they contain. Moreover, scientists applied 

 themselves at an early date to purely palaeontologic investiga- 

 tions, several described from a given geologic division all the 

 material known up to their time, while others devoted themselves 

 to the study of a single group. It will be easily understood that 

 we can not here cite names. The period of which we are speak- 

 ing is closely connected with t"he contemporary period, when 

 researches are becoming more and more numerous. In the 

 beginning of his great work, entitled Enchainements du rnqride 

 animal^ 'M. G-audry enumerates, by the side of each group, the 

 names of the scientists who contributed most to make it known. 

 *' Although these lists contain more than 500 citations, they are 

 far from being complete. . . I should never finish, did I 

 undertake to recount all the intellectual labors that have been 

 expended since the death of Cuvier, to bring to light the genera- 

 tions of living creatures which existed in days gone by." 



We shall have occasion, in the course of the systematic part 

 of this work, to mention the most important of those labors. 



The problem of the origin of variations in faunas continued 

 to preoccupy the most eminent geologists. The prevailing opin 

 ion was that species are absolutely characteristic of the horizons 

 in which they are found, and that no one of them passes from 

 one formation to another. In 1850, d'Obbigny, following the ideas 



