THE RULES OF NOMENCLATURE 355 



for the moment, the Animal Kingdom is divided into 

 a number of phyla, each of which is divided into classes, 

 these again being divided into orders, these into families, 

 these into genera, and these into species. To define a 

 species is difficult enough among living animals, where 

 the test of inter-breeding is available ; among fossils 

 there is only one really scientific way of delimiting 

 species, and that is by determining whether variation is 

 continuous or discontinuous. To do this a large number 

 of individuals from the same bed must be taken, note 

 made of those measurable characters in which they show 

 variation, and a series of accurate measurements taken. 

 The results for each character are plotted as a graph, the 

 horizontal coordinates being the measurements and the 

 vertical coordinates the number of individuals giving 

 each particular measurement. If the plotted curve is a 

 simple one with a single maximum, that is evidence for 

 treating all the individuals as one species ; but if it 

 shows two or more maxima with very decided minima 

 between, then there is ground for regarding them as two 

 or more species. Confirmation must be sought in the 

 curves for the other measurable characters.* 



This method is only practicable when large numbers 

 of individuals from the same bed are available, and 

 when their variations are expressible in simple numerical 

 form. In the absence of such conditions no better 

 definition of a species than this can be suggested for the 

 purposes of palaeontology : A species is a collection of 

 individuals so nearly alike that they may conveniently be 

 denoted by the same name. This definition leaves the 

 decision as to whether there is sufficient likeness to the 

 judgment of every palaeontologist, and judgments will 



* For a good illustration of this method see Hickling, G., 

 "Variation of Planorbis multiformis Bronn," Mem. Proc. Manchester 

 Lit. Phil. Soc., vol. Ivii., pt. 3 (1913). 



