196 Prof. J. D. Dana on Cephalization. 



of Megasthenes and Microsthenes, among Mammals^ we cannot 

 look for the marked diversity that subsists between Decapods 

 and Tetradecapods, although the naturalness of the subdivisions 

 is none the less real. The words Megoicephals and Micrencephals 

 (signifying large-hrained and small-brained Mammals) may better 

 satisfy the desire for names expressing something tangible in 

 the structure. Yet they do not appear to indicate the funda- 

 mental distinction between the groups. A general structural 

 characteristic may yet be detected corresponding to these mega- 

 sthenic and microsthenic qualities ; but even then the distinctive 

 idea of the subdivisions could hardly be better expressed than 

 by the names proposed. 



The parallelism between the Megasthenes and Microsthenes 

 among Mammals, and the Decapods and Tetradecapods among 

 Crustaceans;, suggests that if the subdivisions be called orders in 

 the latter case, they should be so called in the former. 



The distinction between Megasthenes and Microsthenes may 

 perhaps become more intelligible if we regard a living structure 

 as a Irfe-system, ov, speaking dynamically, a life-hattery. In order 

 that such batteries may have a very wide range of size^ two or 

 more plans of construction, more or less different, appear to be 

 requisite. With one plan, there is a certain magnitude which is 

 that of most efficient action and power ; and from this magni- 

 tude there may be a series of larger and smaller sizes, reaching 

 to the outer limits of normal perfection, and then, if these 

 limits be passed in either direction (that is, either on the side of 

 too great magnitude or of too little), degradation in the structure 

 and its powers begin to appear. 



To carry the species through another range of sizes, with 

 normal pei'fection of structure, another somewhat different plan 

 is required. The Megasthenes represent one such plan, the 

 Microsthenes another. 



This idea is brought out by the writer in his chapter on the 

 Classification of Crustaceans already referred to. He there says, 

 speaking of the orders of Crustaceans, viz. Decapods, Tetra- 

 decapods, and Entomostracans : — 



" I. Each type corresponds to a certain system offeree more or less 

 centralized in the organism, and is an expression of that force, — the 

 higher degree beingsuchas is fitted forthe higher structures developed, 

 the lower such as is fitted for structures of inferior grade and size. In 

 other words, the life-system is of different orders for the different 

 types, and the structures formed exhibit the extent of their spheres 

 of action, being such as are adapted to use the force most effectively, 

 in accordance with the end of the species. 



"II. In a given type, as the first, for example, the same system 

 may be of different dimensions, adapted to structures of different 



