NOMENCLATURE. 251 



inferior importance peculiar to the subgeneric group. Sub- 

 genera may thus be regarded as incipient genera. Families 

 are groups of allied genera possessing in common some char- 

 acter (generall}' structural) ; and subfamilies are distinguished 

 by somewhat inferior but peculiar characteristics from the 

 families. Families usually i-eceive the name of the principal 

 genus with the termination idf^. Thus the genus Helix belongs 

 to the family Helicida?. Subfamilies take the termination inse 

 with the name of the principal genus : thus we have the family 

 Muricid?e, with the subfamilies Muricinee and PurpurinfB for 

 the groups of genera typified by Murex and Purpura. Sub- 

 orders, orders, subclasses and classes are assemblages similarly 

 constituted ; onlj'^ in each step made, the characters pervade a 

 larger group of species, etc., and thus become more and more 

 important in the structure and economy of the animal. 



In an ideal classification each group of similar systematic 

 value would possess structural characters of equal importance. 

 If such military order and subordination existed in nature, it 

 might readily be perverted by our want of perception and judg- 

 ment ; but many naturalists have become satisfied that the same 

 laws which have produced variation in the individual, work to 

 produce variation in every characteristic, be it of minor or 

 major importance, and therefore the sharp lines of demarkation, 

 indicated by the systematic scheme do not exist in nature, they 

 are fictions necessary in classification, for the purpose of indi- 

 cating certain agreed-upon stages of a continuous chain of 

 differentiation. Suflficient evidence has accumulated from the 

 study of palseontology, embryology and comparative anatomy to 

 fully sustain this evolutionary idea of nature, as to most of the 

 inferior systematic divisions, but the evidence is still insuflficient 

 to show conclusively the evolution of orders and classes within 

 the subkingdom Mollusca, or of that subkingdom with the 

 others from some common ancestral type. The classes and 

 principal orders of the mollusca exhibit their structural chai-ac- 

 teristics (so far as these can be indicated by the shell and other 

 preserved portions of the animal) from their first geological 

 appearance. It may be added that certain genera have main- 

 tained these original characters from the older fossiliferous 

 deposits to the present time. 



" Great difficulty has always been found in placing groups 

 according to their affinities. This cannot be effected in — the 

 way in which we are compelled to describe them — a single 

 series ; for each group is related to all the rest ; and if we 

 extend the representation of the affinities to very small groups, 

 any arrangement on a plane surface would fail, for the affinities 

 radiate in all directions, and the ' network ' to which Fabricius 

 likened them, is as insuflScient a comparison as the ' chain ' of 

 older writers." — WooitWARD. 



