MILNE-EDWARDS 199 



suppose that Nature wishes to adapt a fish, which breathes by 

 gills, to life in the air ; she does not create an organ specially 

 for this purpose, but utilises the moist gill-chamber (e.g., 

 in Anabas scandens], modifying it in certain ways so that the 

 fish can take advantage of the oxygen it contains. But this 

 gill-chamber lung is at best a makeshift, and when she comes* 

 to the more definitely terrestrial Amphibia Nature gives up 

 the attempt to use the gill-chamber as a lung, and creates a 

 new organ, the true vertebrate lung, specially adapted 

 for breathing air (p. 475). 



But whatever means Nature adopts, her aim is always the 

 same to specialise, to differentiate, to produce diversity 

 from uniformity. 



Differentiation not only raises the level of organisation ; 

 it usually also takes the direction of adaptation to particular 

 habits of life, and this is perhaps the most fruitful cause 

 of diversity. Everywhere we find animals specialised in 

 adaptation to their environment to life in air or water, or 

 on land and many of their most striking differences are due 

 to this cause. But adaptation may also act in reducing 

 diversity, for there necessarily occur many instances of 

 parallel adaptation or convergence. So we get the ex- 

 traordinary parallelism between the families of marsupials 

 and the orders of placentals, 1 the remarkable similarity 

 between the respiratory organs of land-crabs and air- 

 breathing fish to mention only two out of an immense 

 range of analogous facts. 



The last cause of diversity that Milne-Edwards adduces 

 is what he calls a " borrowing " of peculiarities of structure 

 from another systematic group. Thus, " among reptiles, 

 the tortoises seem to have borrowed from birds some of their 

 characteristic features of organisation ; and among the 

 sauroid fishes the piscine type seems to have been influenced 

 by the type from which reptiles are derived " (p. 479). So 

 many riddles that, a little later on, stimulated the ingenuity 

 of the evolutionists ! 



Such, then, were the factors which Milne-Edwards 



'Studied by Isidore Geoffrey St Hilaire in his paper Classification 

 parallclique des Maniiniferes, C. R. Acad. Set., xx., 1845. Remarked 

 upon by Cuvier, Rcgne animal. , i., p. 171, 1817, also by de Blainville. 



O 



