Mr. T. Davidson on Recent Terebratulse. 29 



1 General Introduction' one year later. Prof. Suess has adopted 

 my classification in principle, but does not distinguish the sub- 

 genera from the genera, as those two sorts of division have ap- 

 peared to him to possess a similar value; and he may very pos- 

 sibly be correct, although it appears to be at present a matter 

 of opinion, since many naturalists prefer the one to the other 

 system. Mr. Reeve has to a certain extent made use of the sub- 

 generic arrangement ; and my reason for preferring the retention 

 (at least provisionally) of subgenera is based on the in all pro- 

 bability erroneous idea that genera and subgenera are not of 

 equal value. Our record in this respect (as Darwin would so 

 justly observe) is very incomplete. To be able to divide and 

 subdivide the species definitely (if such were possible), one 

 would require to possess a knowledge of the animals of all 

 the recent and fossil forms, or, in default of that impossibility, 

 to be perfectly acquainted with at least the interior dispositions, 

 calcareous processes, or imprints left by the animal in the va- 

 rious species we require to class; and then one would be able to 

 estimate, with an approximate degree of certitude, the value of 

 the interior characters upon which our subdivisions are founded; 

 and it is probable, from the rapid march of discovery, that the 

 day will arrive when the interior dispositions of the larger num- 

 ber of species will have been examined and placed in comparison 

 one with the other. The result of this will in all probability be, 

 that the differences we now think so much of will be singu- 

 larly attenuated in their importance, so much so that all the so- 

 termed genera, subgenera, and species will become very much, 

 although never entirely, connected by a series of modifications, 

 which would be even closer had the geological and palseonto- 

 logical record been more perfectly preserved; for many fossil 

 forms or intermediate links are, no doubt (as Darwin has so 

 beautifully explained in chapters ix. and x. of the third edition 

 of his admirable work upon the ' Origin of Species '), irrecover- 

 ably lost to us*. Exceptions to a general rule in natural history 

 are oftentimes awkward subjects to be dealt with, and these do, 

 alas ! very often occur in our superstructure. Thus, for example, 

 the genus Terebratella of D'Orbigny was founded for the recep- 

 tion of those species of Terebratula in which the loop is doubly 

 attached, first to the hinge-plate, and afterwards to the mesial 



* " That the extinct forms of life help to fill up the wide intervals be- 

 tween existing genera, families, and orders, cannot be disputed. For if we 

 confine our attention either to the living or to the extinct alone, the series 



is far less perfect than if we combine both into one general system that 



in a perfectly natural classification many fossil species would have to stand 

 between living species, and some extinct genera between living genera, even 

 between genera belonging to extinct families " (pp. 356, 357). 



