ZOOLOGY AND PALAEONTOLOGY 191 



It will be seen, by this very simple table, that the 

 species and genera of existing nature or, what 

 comes to the same thing, those of each of the faunas 

 which succeeded each other in the life of the 

 earth comprise a horizontal line drawn through 

 a series of branches evolving on parallel lines, and 

 most often without any known point of contact. 

 From this there results a relatively very large 

 facility in establishing the natural divisions. 



When it is a question, on the contrary, of phyletic 

 branches, we have to deal with vertical lines com- 

 posed of a series of forms closely related by means 

 of direct descent, and connected from stratum to 

 stratum by imperceptible transitions. We cannot, 

 therefore, wonder at the difficulties, nay, more, at 

 the impossibility which palaeontologists encounter in 

 establishing divisions of which the demarcations 

 have become artificial and fleeting, and are contrary 

 to the very essence of the constitution of these 

 branches. There is, in my opinion, no way out 

 of this difficulty but to adopt resolutely a nomen- 

 clature different from the zoological nomenclature. 

 This was well understood by Waagen and Neumayr 

 when they proposed to substitute for the term 

 species that of ascending or descending mutation. 

 Being no longer embarrassed by comparisons with 

 the value of the actual species, there is nothing to 

 prevent a distinguishing name from being given to 



