212 Prof. J. D. Dana on the Classification of Mammals, 



in the normal magnitncle of the life-system. The Decapods are 

 built on a life-system of large size as to plan as compared with 

 that of the Tetradecapods. Deducing the relative size from the 

 mean dimensions of the active species under the two types, the 

 ratio is nearly as 4 : 1. (See the papers of the author already 

 referred to.) Moreover, while thus distinct, the subdivisions of 

 the two orders form parallel series, — the Brachyurans,'Anomou- 

 rans and Macrourans running a close parallel with the Isopods, 

 Anisopods and Amphipods ; for the Isopods are literally Bra- 

 chyural Tetradecapods, and the Amphipods Macroural^. 



The life-system in the Entomostracans is on a still smaller 

 plan. 



Among the viviparous Mammals (exclusive of Man) the first 

 group differs from the second on this same principle — the fact 

 of a larger and more powerful type of structure or life-system. 

 This fact stands out boldly to view on comparing active species 

 of each — the orang-outang with the largest bat, the tiger with 

 any Insectivorc, the horse or elk w ith any Rodent, a Cetacean 

 with any Edentate. The species of the second division are rela- 

 tively small and feeble animals ; and if they are sometimes of 

 great bulk, as some ancient sloths, it is an example, though 

 natural to the species, of vegetative overgrowth ; for the bodies 

 of the sloths, great and small, are, in fact, too bulky to be 

 wielded well by the small life-system within. 



Adopting this view as presenting the true basis for the sub- 

 division of the viviparous Mammals, the two groups are sig- 

 nificantly designated (1) Megasthenes (from fxejaiff great, and 

 adeva, strength), and (2) Microsthenes (from /xcKpo^i, small, and 

 aOevos;)- Judging of the mean size of the life-system in the 

 two divisions from their more active as well as powerful species, 

 the lineal ratio is not far from 4:1, as between the Decapods 

 and Tetradecapods. 



The orders in these two groups, the Megasthenes and Micro- 

 sthenes, have throughout a precise parallelism. The Bats or 

 Chiropters in the latter represent the Monkeys or Quadrumanes 

 in the former, these orders having such close relations that they 

 are made to follow one another in Cuvier's system; the Insecti- 

 vores represent the Carnivores ; the Kodents represent the Herbi- 

 vores : and the Brutes or Edentates the Mutilates. 



* The parallelism is complete ; for the Amphipods differ from the Iso- 

 pods just as the Macrom-ans from the Brachyurans, in having a larger and 

 less compacted head, looser and larger mouth-organs, longer segments to 

 the body, and an elongated foot -bearing abdomen — all points of inferior 

 concentration and cephahzation. 



