XVI INTRODUCTION. 



[CH. 



class by characters which others do not share in common 

 with it. Mr. Alder is at present occupied with the sub- 

 ject of the British Tunicata ; and it will doubtless re- 

 ceive from that talented and experienced naturalist the 

 same elucidation as was bestowed on his celebrated 

 Monograph of our native Nudibranchs. 



Species. — I now venture to offer a few remarks on a 

 very difficult and perhaps insoluble prolilem, viz. the 

 nature of species. The difficulty of this investigation is 

 greater in the study of the Mollusca and other Inver- 

 tc])rata tlian in that of more highly organized animals, 

 because one characteristic element, from the nature of 

 their reproductive system, is here wanting or beyond the 

 reach of observation. Nearly all the land Mollusca, 

 the habits of which it is comparatively easy to study, 

 have botli sexes united in the same individual ; and not 

 even the acpiarium will enable us to make those experi- 

 ments as to the fertility or sterility of hybrids to which 

 such importance is attached in the discussion of this 

 (|uestion in the case of vertebrate animals. 



The forms of some shells appear to be more perma- 

 nent oi- cai)al)le of being reproduced without any modi- 

 fication than others. The Silurian Linyula, which claims 

 the precedence of all Mollusca in point of antiquity, is 

 said to be undistinguishable from an existing species; 

 and its mould must tliercfore have been continued from 

 the womb of time to the present day without the slight- 

 est change. The secondary strata contain many well- 

 known instances of a similar persistence of form, espe- 

 cially those of Terebratula cuput-serpentis and some 

 Foraminifera, which are considered by competent autho- 

 rities not to difier from species which now live in the 

 adjacent seas. A large proportion of the fossil shells 

 found ill the lowermost of tlie Pliocene strata (or 



