192 NATURAL SCIENCE. March. 



" a very extensive island," though he doubts whether this island ever 

 formed part of a continent. 



Turning to their zoology, we find that they are entirely destitute 

 of mammals, but that they possess Amphibia, having two species of 

 frogs and three species of Caecilia, snake-like creatures, which 

 burrow underground in the manner of worms. Now, it seems impos- 

 sible to explain the presence of these Amphibia unless, at some 

 remote period, the islands formed part of a continent, for salt-water 

 is fatal to them, and destroys even the eggs of frogs. On the other 

 hand, if the former connection with a continent be admitted, what 

 can be said of the absence of mammals, for one would have expected 

 that some of the smaller genera, such as rats, mice, civets, lemurs, 

 and insectivores would have survived. Dr. Wallace suggests that the 

 islands have at some time been so nearly submerged that the portions 

 remaining above water were too small to support the existence of the 

 smallest mammals. If this be the explanation, and if we accept the 

 evidence of the Amphibia as to ancient continental connection, then 

 the absence of mammals in such islands cannot be taken as proof 

 that they have never been part of continental land. 



New Caledonia, again, in the Pacific, is regarded by Dr. Wallace 

 as an oceanic island. There are no indigenous Mammalia or Am- 

 phibia, the solitary frog being known to have been introduced. There 

 are several peculiar lizards and a snake (one of the Boas) ; and 

 the island is separated from neighbouring groups by water of more 

 than I, ooo fathoms deep. Notwithstanding this limited vertebrate fauna 

 there is evidence that New Caledonia has once been part of an exten- 

 sive land area. Stratified rocks, believed to be of Secondary and 

 Tertiary age, enter into its geological structure, and there is a genus of 

 land snails (Placostyliis) which occurs in the neighbouring archipelagoes, 

 as well as on Lord Howe's Island and in New Zealand. Hence it 

 has been recently argued that the unity and limitation of the Placostylus 

 area can only be explained by the supposition that these islands are 

 portions of a broken-up and submerged continent which was 

 distinct from Australia. To reconcile such a supposition with the 

 absence of mammals, it is only necessary to assume that this continent 

 dates back to a time when mammals were not in existence in the 

 Pacific region, just as Australia dates from a time when only marsu- 

 pial mammals were in existence. 



The cases above mentioned show that there is no constant relation 

 between the geological structure of oceanic islands and the manner in 

 which they have received their present inhabitants, for there are 

 islands which are not far from a continent, and have clearly formed part 

 of the continental area within the limits of Tertiary time, and yet are 

 without any indigenous Mammalia or Amphibia. There are other 

 islands which are destitute of Mammalia and are geographically 

 oceanic, but nevertheless, have a geological structure of continental 

 type. In other words, there are islands which testify to the former 



