514 HISTORY OF CONCHOLOGY. 



lusca he clearly saw the impropriety of making the presence 

 or absence of the shell an ordinal character ; and he knew, 

 vaguely it may be, the affinity between the bivalvular Mol- 

 lusca and the Tunicata. "For what" we translate his 

 words " are the Testacea but Mollusca furnished with a 

 shell, and what are Mollusca but Testacea destitute of it ? 

 There is the most exact agreement of the tenants of the uni- 

 valve shells which are called Helices with the naked slugs ; 

 and an agreement not to be overlooked of bivalves with the 

 Ascidia ; and the very error of our predecessors, who said 

 that slugs were merely snails which had crept out of their 

 shells, proves their near affinity. Besides the insensible but 

 evident transition of nature from the naked Limax to the 

 testaceous passing from the former, which at most has the 

 mere rudiment of an internal shell to the latter by means of 

 the Buccinum (Limneus glutinosus), which conceals its 

 membranous shell under a fleshy mantle, supports plainly 

 our opinion. Therefore I do not doubt that a future age 

 will join together the naked slugs and the shelled snails, 

 which authors have separated into different orders." " If we 

 wish," he writes in another place, " properly to know and 

 discriminate natural objects, they must be considered in every 

 point of view and in all states, so far as human imbecility will 

 permit. The attainment of knowledge is thus indeed ren- 

 dered more difficult, but at the same time more pleasant and 

 accurate ; genera indeed are multiplied, but by this way only, 

 if by any, can species ever be determined. This is the alpha 

 and omega of our labours, since systems and methods and 

 genera are arbitrary and framed by the narrow limits of our 

 knowledge. Nature acknowledges one division of created 

 bodies only the living and brute matter spurning for the 

 most part the arrangements of systematists into classes and 

 orders, families and genera, and her productions are often so 

 affined that their limits can never be strictly fixed. Cha- 

 racters derived from the interior and exterior structure of 

 bodies deceive us not solely in the higher divisions ; and even 

 the manner of life and the mode of propagation do not afford 

 any certain distinctions either in those races which are visible 

 or in those which are invisible to the naked eye. There is, 

 therefore, only one family, and one Father of all, who has 

 marked with a constant character all species whatever from 

 the Monad to the turret-bearing Elephant, and has distin- 

 guished Man alone with a reasonable soul." * 



The celebrated Pallas was another who at this period had 



* See the Prsefatio to his Verm. Ter. et Fluv. vol. i. 1773- 



