158 ESSAY ON CLASSIFICATION 



combined, if form can be shown to be characteristic of families. The 

 same is the case with genera and species; and nothing is more remote 

 from the truth than the idea that a genus is better defined in pro- 

 portion as it contains a greater number of species, or that it may be 

 necessary to know several species of a genus before its existence can 

 be fully ascertained, A genus may be more satisfactorily characterized, 

 its peculiarity more fully ascertained, its limits better defined, when 

 we know all its representatives; but I am satisfied that any natural 

 genus may be at least pointed out, however numerous its species may 

 be, from the examination of any single one of them. Moreover, the 

 number of genera, both in the animal and vegetable kingdom, which 

 contain but a single species is so great that it is a matter of necessity 

 in all these cases to ascertain their generic characteristics from that 

 one species. Again, such species require to be characterized with as 

 much precision, and their specific characters to be described with as 

 much minuteness, as if a host of them, but not yet known, existed 

 besides. It is a very objectionable practice among zoologists and 

 botanists to remain satisfied in such cases with characterizing the 

 genus, and perhaps to believe what some writers have actually stated 

 distinctly, that in such cases generic and specific characters are iden- 

 tical. 



Such being the natural relations and the subordination of types, 

 classes, orders, families, genera, and species, I believe, nevertheless, 

 that neither types, nor classes (orders of course not at all) nor fami- 

 lies, nor genera, nor species have the same standing when compared 

 among themselves. But this does not in the least interfere with the 

 prominent features of orders, for the relative standing of types, or 

 classes, or families, or genera, or species does not depend upon the 

 degrees of complication of their structures as that of orders does, but 

 upon other features, as I will now show. The four great types or 

 branches of the animal kingdom, characterized as they are by four 

 different plans of structure, will each stand higher or lower, as the 

 plan itself bears a higher or lower character, and that this may be 

 the case we need only compare Vertebrata and Radiata.^*' The dif- 

 ferent classes of one type will stand higher or lower, as the ways in 

 which and the means with which the plan of the type to which they 



" I must leave out the details of such comparisons, as a mere mention of the point 

 suffices to suggest them; moreover, any text-book of comparative anatomy may furnish 

 the complete evidence to that effect. 



