FAMILIES. 241 



Of all the natural groups in the animal kingdom there 

 remain, then, only families and orders, for the distinction 

 of which form can apply as an essential criterion. But 

 these two kinds of groups are just those upon which 

 zoologists are least agreed; so that it may not be easy to 

 find a division which all naturalists would agree to t t ake 

 as an example of a natural order. Let us, however, do 

 our best to settle the difficulty, and suppose, for a moment, 

 that what has been said above respecting the orders is 

 well founded, that orders are natural groups character- 

 ized by the degree of complication of their structure, and 

 expressing the respective rank of these groups in their 

 class, then we shall find less difficulty in pointing out 

 some few groups which would be generally considered as 

 orders. I suppose most naturalists would agree, for in- 

 stance, that among Reptiles the Chelonians constitute a 

 natural order ; that among Fishes, Sharks and Skates 

 constitute an order also; and, if any one would urge the 

 necessity of associating also the Cyclostomes with them, 

 it would only the better serve my purpose. The Ganoids, 

 even when circumscribed within narrower limits than those 

 I have assigned to them, and perhaps reduced to the ex- 

 treme limits proposed for them by J. Mtiller, I am equally 

 prepared to take as an example, though I have in reality 

 still some objections to this limitation, which, however, 

 do not interfere with my present object. The Decapods, 

 among the Crustacea, I suppose everybody would also 

 admit as an order; and I do not care here what other 

 families are claimed, besides the Decapods, to complete the 

 highest order of the Crustacea. Among the Acephala, I 

 trust, the Bryozoa, Tunicata, Brachiopods, and Lamelli- 

 branchiata would be also very generally considered to be 

 natural orders. Among the Echinoderms I suppose the 



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