The Peinciples of Palaeontology. 145 



It was Lamarck who first conceived the idea that species are 

 not immutable but were derived from one another. Philosophers, 

 such as Chambers, G. Saint-Hilaire, Grant, von Buch and some 

 others, considered by Darwin and Wallace as the precursors of 

 transformism, occupied themselves in demonstrating, not the non- 

 existence of species, but their variability. We know that Darwin 

 and Wallat;e were the first who accumulated numerous prooft 

 of variation in living species and demonstrated how arbitrary as 

 times is the distinction made between species, races and varieties. 



In Palaeontology the problem of the limitation of species is still 

 more difficult. We no longer have at our service the criterion, 

 which, moreover, is rarely used in Biology, that is the sterility of 

 hybrids, as a proof of difference in species ; and we have no 

 means of ascertaining whether the individuals observed come 

 from the same parents. We are forced then to take for our 

 guidance the principle of continuity; we bring together in one 

 species those individuals which resemble each other in essential 

 characters and differ only in secondary characters, such as 

 among living creatures determine varieties and races. We 

 appreciate how profoundly arbitrary such a definition is. One 

 variation may seem fundamental to some palaeontologists while 

 to others it may not appear of sufficient consequence to justify 

 the creation of distinct species. The particular tendencies of 

 each one will exert their influence, just as in Zoology and 

 Bjtany the discussions up to a recent epoch have been incessant 

 •between students who were inclined to multiply specific denomi- 

 nations and those who, on the contrary, wished to restrict them. 



Practicall}^, we generally endeavor to group under the same 

 specific denomination those forms which show among themselves 

 gradual transitions. Wnen the passage forms are wanting we 

 make a division. This presupposes that we have under consider- 

 ation a considerable quantity of material, whilst in fact most 

 species were originally established after the examination of but 

 few specimens, and this is the case now in regard to many new 

 species. 



Continuous series of forms.— While the examination of these 

 questions is the order of the day, biologists are constantly dis- 

 covering many instances of continuous transitions between 

 extreme forms which have hitherto been considered very dis 

 19 



