444 ABSENCE OF MAMMALIA IN ISLANDS. CHAP. xxn. 



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

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

 out at sea. Unquestionably, the total exclusion of quadru 

 peds in general, which could only reach such isolated habita 

 tions by swimming, seems to imply that nature does not 

 dispense 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 squirrels, 

 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 Upper Mrocene 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, clausiliay &c.), for the most part 

 different or proper to each island. About 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 distinct, 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 disappeared from 

 the fauna of the Madeiran Archipelago, though still extant in 

 Africa and Europe. Many which were amongst the most 

 common in the Newer Pliocene period, have now become the 



