Manchester Memoirs, Vol.lvii. (191 3), No. 10- 19 



off as a distinct species, the ancestral form having parted 

 company with the carinate and turreted varieties. The 

 partition in this case serves to emphasise the united 

 character of the remaining forms. Of far greater import- 

 ance, however, is the fact, as it appears to me, that by 

 such partitions, and in no other way, can a species be 

 defined. Herein Hes the whole distinction between the 

 treatment of these shells now given and that adopted by 

 Hyatt. He assumes that any form recognisably dis- 

 tinguished from another may be designated a distinct 

 species. On this assumption any person is at libert}\ 

 among a series of forms, to distinguish as many or as few 

 species as he chooses, since the "species" is an abstraction 

 with no objective existence. But the aim of classification 

 is to indicate the branches of the zoological tree, of which 

 the ultimate branches are species. In other words, no 

 form can rank as a species until it becomes biologically 

 independent. It has already been hinted that the strongly 

 carinate discoid forms here described seem to show a 

 tendency to break off as the earlier type has done, but 

 until a definite discontinuity appears, they cannot, in my 

 opinion, be regarded as constituting a distinct species 



Two observations may be made as to the bearing 

 of this study on the use of palaeontological data for 

 stratigraphical purposes. Firstly, as we have just seen, 

 a series of organisms may form an evolutionary chain and 

 appear in stratigraphical sequence, yet the members of the 

 series may not be diagnostic of successive zones, owing to 

 the persistence, as here described, of the earlier types, 

 conjointly with their modified descendants. In such a 

 case, the horizon clearly cannot be determined, except by 

 a complete suite of specimens, sufficient to determine the 

 real stage of evolution reached. An individual specimen 

 will only serve to determine that the horizon cannot be 



