1866.] JUKES OLD BED SANDSTONE AND DEVONIAN. 371 



since a species founded on a single specimen, or a few rarely occur- 

 ring specimens, counts as much in this comparison as a species of 

 which specimens occur everywhere in the greatest profusion. 



The abundance of individuals must be an element of importance 

 towards the establishment of a species, as enabling the palaeontolo- 

 gist to include all its varieties ; but, even in two equally distinct 

 species, it appears to me that richness in individuals and extent of 

 diffusion ought to be taken into account by the geologist, when esti- 

 mating ih.eiv potency in chronological significance. 



Tables of percentage in which abundance and diffusion of species 

 are not taken into account are really of no great value. I strongly 

 suspect that the ninety-two carboniferous species mentioned above, 

 include those which are at the same time most abundant and most 

 widely diffused in both groups of rock, while the rare and doubtful 

 species will be found entirely among the remainder. Some of the 

 latter, too, will eventually be decided to be mere varieties of the 

 former, and will therefore be not only erased from the one list, but 

 added to the other. 



