444 ABSENCE OF MAMMALIA IN ISLANDS, CHAP. XXll. 



exceptions to the rule, might have made their way to distant 

 ishxnds by flight, for they are often met with on the wing far 

 out at sea. Unquestionably, the total exclusion of quadru- 

 peds in general, Avhich could only i*each such isolated habita- 

 tions by swimming, seems to imply that nature does not dis- 

 pense with the ordinary laws of reproduction when she 

 peoples the earth with new forms; for if causes purely imma- 

 terial were alone at work, we might naturally look for squir- 

 rels, rabbits, polecats, and other small vegetable feeders and 

 beasts of prey, as often as for bats, in the spots alluded to. 



On the other hand, I have found it difficult to reconcile 

 the antiquity of certain islands, such as those of the Madeiran 

 Archipelago, and those of still larger size in the Canaries, 

 with the total absence of small indigenous quadrupeds; for, 

 judging by ancient deposits of littoral shells, now raised 

 high above the level of the sea, several of these volcanic 

 islands (Porto Santo and the Grand Canary among others) 

 must have existed ever since the Ui)per Miocene period. 

 But, waiving all such claims to antiquity, it is at least 

 certain that, since the close of the Newer Pliocene period, 

 Madeira and Porto Santo have constituted two separate 

 islands, each in sight of the other, and each inhabited by an 

 assemblage of land-shells {helix, pupa, clausilia, &c.) for the 

 most part diiferent or proper to each island. Aboait thirty- 

 two fossil species have been obtained in Madeira, and forty- 

 two in Porto Santo, only five of the whole being common to 

 both islands. In each the living land-shells are equally dis- 

 tinct, and correspond, for the most part, with the species 

 found fossil in each island respectively. 



Among the seventy-two species, two or three appear to be 

 entirely extinct, and a larger number have di8ap2>eared from 

 the fauna of the Madeiran Archipelago, though still extant 

 in Africa and Europe. Man}" which were among the most 

 common in the Newer Pliocene period have now become the 



