Notices of New Books. 7603 



with those of the perfected species, are the causes assigned for the 

 absence of geological evidence of the gradation of one specific form 

 into a higher. 



Another difficulty cited by Mr. Darwin is the existence in a 

 particular group of species adapted to fill places in the economy 

 of nature, for which the group in its entirely was not designed; for 

 instance, the adaptation of the bat for flight, though its nearest allies 

 have all terrestrial habits, and the aquatic Carnivoras, structurally 

 allied to animals of totally distinct habits, present difficulties in the 

 way of community of descent, because of the apparent impossibility 

 of the existence of the animal in an intermediate state, and of the 

 improbability of natural selection leading to characters and habits 

 inconsistent with those habits which have tended to the well-being 

 of the group. Mr. Darwin meets the difficulty by citing sundiy cases 

 of species possessing extended diversity of habits and small measures 

 of organic modification to suit those habits, — of animals of structures 

 apparently adapted to particular habits filling a distinct position in 

 the economy of nature ; for instance, a species of woodpecker, which 

 is obviously adapted by its character for climbing trees and licking 

 out insects with its long tongue from the crevices of the bark, existing 

 in the treeless American prairies, — of the Musula vison of North 

 America, which has webbed feet, and resembles the otter in its fur, 

 short legs and long tail, which in summer preys on fish, but during 

 the frozen winter preys like other polecats or mice and other land 

 animals, — of the family of squirrels, with every intermediate grade of 

 character, from a merely flattened tail to the flying squirrels, having 

 their tail and limbs united by a broad expanse of membrane, — of a 

 flying Lemur, with such a fully developed similar membrane, as to have 

 been atone time mistaken for a bat, — of the loggerheaded duck, whose 

 wings do not fulfil the ordinary fiinctions, but are used as propelling 

 fins in the water, and as a kind of front legs on the land, — of the grebes 

 with feet intermediate in character between the normal type of webbed 

 feet and the feet of land birds, — of the ostrich, whose wings are only 

 used as sails, — and of the Apteryx, where the wings are utterly func- 

 tionless ; all which instances are looked upon by Mr. Darwin as 

 incipient stages that might ultimately be developed into those greater 

 differences of habit and " adaptive " structure which so widely divide 

 some organic classes without disassociating them as natural groups. 



The difficulty of attributing the formation of organs of no apparent 

 influence in the well-being of its possessors, as the result of natural 

 selection, aflfects the question doubly; first, there would be no apparent 



