VARIATION IN SPACE IN GEOLOGICAL TIMES 143 



has before him only subjects with very clear and 

 distinct characteristics, and will describe them as 

 so many different species, thus leaving it to chance 

 to declare which form shall be distinguished by 

 a specific name out of all the other possible forms 

 of the same species. Or, on the contrary, he is 

 dealing with a very rich deposit, which allows him 

 to gather together some hundreds of subjects 

 grouping themselves round an average type. The 

 most usual process is to arrange all these shells in 

 a continuous series, guided by some characteristic, 

 such as length of the spiral and number of ribs or of 

 rows of tubercules, if dealing, for example, with 

 Gastropods. These characteristics will either be- 

 come less, or, on the contrary, more marked from 

 one end to the other of the series. The most logical 

 process, the one most in conformity with the truth 

 of Nature, would be to draw, side by side, the 

 principal graduated steps in this series, leaving to 

 the whole body a specific name, and taking as 

 varieties the most important modes of variation. 

 This process has the material inconvenience of 

 necessitating very copious and costly illustrations. 

 Palaeontologists have acquired the habit, a sorry 

 one in my idea, of making up in this continuous 

 series a certain number of arbitrary sections, each 

 corresponding to the variation of one or of several 

 characteristics, and of giving a name belonging to 

 a species to each section. It will be seen how much 

 this purely morphological notion of the species in 

 palaeontology differs from the definition at the same 

 time morphological, genetic, and geographical, 

 which we have admitted for the living species. 



