158 ESSAY ON CLASSIFICATION. Pakt I. 



as it is among the Viverriiia, or among the Canina, or among the Bradypodidae, 

 or among the Delphinoida3, etc., etc. We must, therefore, exclude foiTn from the 

 characteristics of natural genera, or at least introduce it only as a modification of 

 the typical form of natural families. 



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 axe least 

 agreed, so that it may not be easy to find a division which all naturalists 

 would agi'ee to take 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 charac- 

 terized by the degree of complication of their structure, and expressing the respec- 

 tive rank of these groups m their class, then we shall find less difficulty in 

 pointing out some few groups which could be generally considered as orders. I 

 suppose most natui'alists would agree, for instance, that among Reptiles the Chelo- 

 nians 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 purposes. Ganoids, even 

 circumscribed within narrower limits than those I had assigned to them, and 

 perhaps reduced to the extreme limits proposed for them by J. Miiller, I am 

 equally prepared to take as an example, though I have in reality still some ojjjec- 

 tions to this limitation, which, however, do not mterfere with my present object. 

 Decapods, among Crustacea, I suppose evei'ybody would also admit as an order, 

 and I do not care here what other families are claimed besides Decapods to com- 

 plete the highest order of Crustacea. Among Acephala, I trust Bryozoa, Tunicata, 

 Brachiopods, and Lamellibi-anchiata would be also very generally considered to be 

 natural orders. Among Echinoderms, I suppose Crinoids, Asterioids, Echinoids, and 

 Holothurioids would be conceded also as such natural orders; among Acalephs the 

 Beroids, and perhaps also Discophorae and Hydroids; while among the Polypi, the 

 Halcyonoids constitute a very natural order when compared with the Actinoids. 



Let us now consider these orders with reference to the characteristic forms they 

 include. The forms of the genuine Testudo, of Trionyx, and of Chelonia are very 

 different, one from the other, and yet few orders are so well circumscribed as that 

 of Chelonians. The whole class of Fishes scarcely exhibits gi-eater differences than 

 those observed in the forms of the common Sharks, the Sawfishes, the common 

 Skates, and the Torpedo, not to speak of the Cyclostomes and Myxinoids, if these 

 famOies were also considered as members of the order of Placoids. Ganoids cannot 

 be circumscribed within narrower limits than those assigned to them by J. Miiller, 

 and yet this order, thus limited, contains forms as heterogeneous as the Sturgeons, 



