LEADING GROUPS OF EXISTING SYSTEMS 145 



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

 branches all such great divisions of the animal kingdom as are con- 

 stituted 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 king- 

 dom; and if there are still differences of opinion among naturalists 

 respecting their limits, let the discussion upon this point be carried 

 on with the iniderstanding that types are to be characterized by dif- 

 ferent plans of structure, and not by special anatomical peculiarities. 

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

 of structure, even thouoh 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 consequence 

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

 us examine this point within limits no longer controvertible. Neither 

 Infusoria nor Intestinal Worms are any longer arranged by com- 

 petent naturalists among Radiata. Why they have been removed 

 may be considered elsewhere; but it was certainly not because they 



'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, de- 

 tails 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 alto- 

 gether by the relative degree of precision and clearness with which I may have ex- 

 pressed 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 information would 

 permit. 



