LECTURES ON MOLLUSCA. 25 



many whirls. The study of mollusks is calculated to warn any stu- 

 dent against hasty generalizations. He is continually finding char- 

 acters important in one family, which prove of little moment in 

 another : marks which he has long rightfully considered coordinate 

 with special distinctions, appearing again in quite different connec- 

 tions, as well as essential differences of animal appearing, where there 

 was nothing in the shell to lead to their suspicion. An artificial clas- 

 sification, therefore,, however convenient as an index to characters and 

 species, does not convey that knowledge of the whole relationships of 

 the animal, which we ought at least to seek to express. It is to he 

 regretted that some of the most learned of modern writers have gone 

 on this artificial plan ; and, from a determination to he guided hy cer- 

 tain special characters as fundamental, have grouped together very 

 unlike creatures, and separated others with natural affinities, to the 

 great perplexing of heginners. Thus, in the arrangement followed 

 at the British Museum, the Grasteropods and bivalves are grouped 

 together, simply because they have a foot; and the Lamp-shells, Ptero- 

 pods, and Cephalopods together, because they have none: the noble 

 Cuttles being degraded to the lowest rank among mollusks ; and two 

 closely allied classes of bivalve shells, as well as the nearly related 

 Gasteropods and Pteropods being separated in the primary division, 

 simply because they have or have not a foot a character which varies 

 to the greatest extent within each separate class ; for many of the 

 Heteropods among the crawlers have not so much of foot as the cut- 

 tles, and the oysters among the bivalves have none at all. The same 

 grouping, according to individual characters, prevails throughout the 

 subordinate divisions. But there is a difference between a classifica- 

 tion and an index. The Linnasan grouping of plants is an admirable 

 index; by consulting which an unknown flower may be at once 

 put into its proper place; but it tells very little, and that little often 

 erroneously, of the true relationships of plants. The " Natural Sys- 

 tem" is much harder to learn, and requires constant alterations; but, 

 so far as it is ascertained, it is a compendium of the existing state of 

 science. So the British Museum method is an admirable index ; for a 

 student, having a fresh animal under examination, can at once ar- 

 range it under its appropriate " Suborder, Tribe, A, a, *, f," &c.; but 

 whether he is showing, or upsetting its true relationships by this pro- 

 cess, is yet to be seen. It was thought in the days of Lamarck that 

 animals, if fully known, might be arranged in a straight line, gradu- 

 ally ascending from the monad to man. Every progress in our dis- 

 coveries impinges upon this idea, and shows that we cannot even 

 arrange by radiations or circles in one plane. We have to branch off 

 into space, like the suns in the universe: the attractions of each, with 

 its attendants in orbits of different planes, being to every other. To 

 express this in a superficial way on paper must needs only give us 

 partial impressions, which nothing but patient study can develop into 

 even an approximation to the truth. 



The comb-gilled crawlers very naturally divide themselves into 

 those with a long retractile proboscis, which can be drawn into the 

 mouth or extended at pleasure ; and those with an external muzzle, 

 more or less produced into a snout. The first group are all preda- 



