142 



ESSAY ON CLASSIFICATION. 



Part I. 



of structure, cast, as it were, into distinct moulds or forms.^ Now there can certainly 

 be no reason why we should not all agree to designate as types or branches 

 all such great divisions of the animal kingdom as are constituted upon a special 

 plan,- if we should find practically that such groups may be traced in nature. 

 Those who may not see them may deny their existence ; those who recognize 

 them may vary in their estimation of their natural limits ; but all can, for the 

 greatest benefit of science, agree to call any group which seems to them to be 

 founded upon a special plan of structure, a type or branch of the animal kingdom ; 

 and if there are still diflerences of opinion among naturalists respecting their limits, 

 let the discussion upon this point be carried on with the understanding that types 

 are to l)e characterized by difierent plans of structure, and not by special anatomical 

 peculiarities. Let us avoid confounding the idea of plan with that of complication 

 of structure, even though Cuvier himself has made this mistake here and there in 

 his classification. 



The best evidence I can produce that the idea of distinct plans of structure 

 is the true pivot upon which the natural limitation of the branches of the animal 

 kingdom is ultimately to turn, lies in the fact that every great improvement, 

 acknowledged by all as such, which these primary divisions have undergone, has 

 consisted in the removal from among each, of such groups as had been placed 

 with them from other considerations than those of a peculiar plan, or in conse- 

 quence of a want of information respecting their true plan of structure. Let us 

 examine this point within limits no longer controvertible. Neither Lifusoria nor 

 Intestinal Worms are any longer arranged by competent naturalists among Eadiata. 

 Why they have been removed, may be considered elsewhere ; but it was certainly 

 not because they were supposed to agree in the plan of their structure with the 



* It would lead me too far were I to consider 

 here the characteristics of the difterent kingdoms of 

 Nature. I may, however, refer to the work of I. 

 Geoffroy St. Hilaire, Histoire naturelle g^nerale 

 des regnes organiques, Paris, 1856, 8vo., who has dis- 

 cussed this subject recently, though I must object to 

 the admission of a distinct kingdom for Man alone. 



^ It is almost superfluous for me to mention here 

 that the terms plan, ways and means, or manner in 

 ■which a plan is carried out, complication of structure, 

 form, details of structure, ultimate structure, relations 

 of individuals, frequently used in the following pages, 

 are taken in a somewhat different sense from their 

 usual meaning, as is always necessary when new 

 views are introduced in a science, and the adoption of 



old expressions, in a somewhat modified sense, is found 

 preferable to framing new ones. I trust the value of 

 the following discussion will be appreciated by its 

 intrinsic merit, tested with a willingness to understand 

 what has been my aim, and not altogether by the rela- 

 tive degree of precision and clearness with which I 

 may have expressed myself, as it is almost impossible, 

 in a first attempt of this kind, to seize at once upon 

 the form best adapted to carry conviction. I wish 

 also to be understood as expressing my views more 

 immediately with reference to the animal kingdom, 

 as I do not feel quite competent to extend the inquiry 

 and the discussion to the vegetable kingdom, though 

 I have occasionally alluded to it, as far as my in- 

 formation would permit. 



