198 AFFINITIES AND GEOGRAPHICAL 



malian type, and that the terrestrial families may all be re- 

 garded as branches of these marine lines, though in some in- 

 stances a passage from one class form to another has taken 

 place on land. Two principles are thus seen at work in the 

 production of the organic tenants of the earth first, a gesta- 

 tive development pressing on through the grades of organiza- 

 tion, and bringing out particular organs necessary for new 

 fields of existence ; secondly, a variative power re-acted upon 

 by external conditions, and working to minor effects, though 

 these may sometimes be hardly distinguishable from the 

 other. Everywhere along the central scale of organization, 

 the land has been, as it were, a temptation or provocation to 

 new and superior forms adapted for inhabiting it. We might 

 almost regard the progression as the result of an aspiration 

 towards new and superior fields of existence, as from the deep 

 sea to the shallow or river-embouchure, from the shore to the 

 bank, from that again to the higher ground in the interior.. 

 He may not yet be held as a very fanciful naturalist who 

 would regard the megatherium as eager to climb the tree 

 which he could only shake, and thus producing a progeny 

 fitted to do that which was the object of his wishes, or the 

 rock-nose whale, which loves to rest its head on rocks beside 

 the beach, as wishful of that mode of life which was at length 

 vouchsafed to a more highly developed descendant. Such too 

 may be found to be the true principle of perfectibility in na- 

 ture a continual, though it may be an irregularly shown 

 tendency to press on to better and better powers, an indefi- 

 nite improveableness, which may work, as in seconds, in the 

 individual, or strike hours in the species. 



The present view of the Genealogical Classification of Ani- 

 mals was nearly completed, when my attention was drawn 

 to certain appearances, as if the arrangement had what may 

 be called a numerical foundation. Within the last twenty- 

 five years, Mr. Macleay originated a theory of this kind, 

 which has been wrought out by Messrs. Swainson, Vigors, 

 and other naturalists. The prevalent opinion amongst them 

 was, that the true divisions and subdivisions were in groups of 

 five : thus, five sub-kingdoms, five classes of vertebrata, five 



