CH. xii.] ADJUSTMENT, DIRECT AND INDIRECT. 59 



the appearance of pterodactyls, the dermal covering of 

 reptiles was very likely as dififereut from that of birds as it 

 is now, so that a reptilian wing could not be formed by a 

 modification of the dermal covering, we find, naturally 

 enough, the wing of the pterodactyl formed, like that of the 

 bat, by a modification of the skeleton. And this fact seems 

 to justify us in the alternative which we have accepted, that 

 the likeness of the pterodactyl to birds is no proof of im- 

 mediate kinship, but only of secondary adaptive variation, 

 as in the case of bats. A similar argument applies to the 

 numerous likenesses between the higher mammals and the 

 marsupials. At an ancient epoch the marsupials were a 

 dominant race of animals, extending all over the world. 

 But since they have been almost everywhere exterminated 

 by their hardier monodelphian descendants, there is no 

 difficulty in the view that direct adaptation to similar differ- 

 ences of environment, when aided by natural selection, has 

 brought about a differentiation of the higher mammals analo- 

 gous to that which had formerly taken place among the 

 marsupials. That six or seven orders of monodelphians 

 should vary in the same direction with six or seven orders of 

 didelphians, is no more surprising than that twenty-nine 

 kinds of American trees should all differ in the same direc- 

 tion from their European congeners. It is certainly far less 

 surprising than would be the simultaneous loss of a pouch 

 and acquirement of a placenta by a host of marsupial genera 

 scattered all over the earth. 



Pursuing the argument a step farther, we may begin to 

 understand, in a general way, eveii the similarity of the eye 

 of a cuttle-fish to the eye of a vertebrate. Utterly unlike a 

 vertebrate in general structure, and so remotely akin that 

 for practical purposes of argument the kinship is of no 

 account, — if a cuttle-fish could be shown to possess numeroua 

 points of special resemblance to a vertebrate, the fact would 

 be an obstacle to any theory of the origin of organic forma. 



