ZOOLOGY AND PALEONTOLOGY 185 



but is sometimes shorter and limited to a few stages. 

 But I have also said that the progress of palseonto- 

 logical discovery tends to lengthen more and more 

 towards the root most of these shortened branches, 

 whose apparently slight longevity can be most often 

 explained by simple gaps in our knowledge. 



Given the constitution of the phyletic branches 

 by a series of mutations passing from one to another 

 by imperceptible transitions, except in the case 

 of series provisionally discontinuous, it is natural 

 to inquire what, in such series, becomes of the limits 

 of the species and of the genus, and whether the 

 palaeontological signification of these two principal 

 terms in nomenclature is in conformity with that 

 generally accepted in zoology. 



Let us first examine the question from the 

 theoretical point of view. Consider A, A 1 , A 2 , A 3 

 . . . ; B, B 1 , B 2 . . . ; C, C 1 . . . ; as a certain number 

 of animal forms taken from the fauna of the present 

 day ; we group these forms in categories which we 

 call species when their important characteristics are 

 the same and they only differ from each other by 

 constant but slight details. When we studied 

 above variation in living nature, we said that 

 species, considered in a broad sense and with- 

 out assigning a specific name to all individual 

 or local varieties, are perfectly independent of 

 each other, and are not connected by transitional 

 forms, save in a few exceptional cases of hybrid- 

 ism. All naturalists are aware that, in general, 

 species arrange themselves naturally into sheaves 

 or groups of species more or less numerous A, A 1 , 

 A 2 , A 3 , etc., separated from another neighbouring 



