3 GO ABSENCE OF TERKESTKIAL MAMMALS Chap. XII. 



two North American species either ref^ularly or occasionally 

 visit Bermuda, at the distance of GOO miles from the main-land. 

 I hear from Mr. Tomes, who has specially studied this family, 

 that many species have enormous ranj^es, and are found on con- 

 tinents and on far-distant islands. Hence we have only to sup- 

 pose that such wandering species have been modified in their 

 new homes in relation to their new position, and we can under- 

 stand the presence of endemic bats on oceanic islands, with the 

 absence of all other terrestrial mammals. 



Another interesting relation exists, namely, between the 

 depth of the sea separating islands from each other or from the 

 nearest continents, and the degree of affinity of their mamma- 

 lian inhabitants. Mr. Windsor Earl has made some striking 

 observations on this head, since greatly extended by Mr. Wal- 

 lace's admirable researches, in regard to the great Malay Ar- 

 chipelago, which is traversed near Celebes by a space of deep 

 ocean, and tliis separates two widely-distinct mammalian fau- 

 nas. On either side the islands stand on a moderately shallow 

 submarine bank, and these are inhabited by the same or by 

 very closely-allied quadrupeds. I have not as yet had time to 

 follow uj") this subject in all quarters of the world ; but, as far 

 as I have gone, the relation generally holds good. For in- 

 stance, Britain is separated by a shallow channel from Europe, 

 and the mammals are the same on both sides ; and so it is with 

 all the islands near the shores of Australia. The West-Indian 

 Islands, on the other hand, stand on a deeply-submerged bank, 

 nearly 1,000 fathoms in depth, and here we find American 

 forms, but the species and even the genera arc quite distinct. 

 As the amount of modification which animals of all kinds un- 

 dergo partly depends on the lapse of time, and as islands sep- 

 arated from each other or from the main-land by shallow chan- 

 nels are more likely to have been continuously united within a 

 recent period than islands separated by deeper channels, we 

 can imdcrstand how it is that a relation exists l)etween the 

 depth of the sea separating two mammalian fiiimas, and the de- 

 gree of their affinity — a relation which is quite inexplicable on 

 the theory of independent acts of creation. 



The foregoing statements in regard to the inhabitants of 

 oceanic islands — namelv, the fewness of the species, with a 

 large proportion consisting of endemic forms — the members 

 of certain groups, and not of other groups in the same class, 

 having hoen modified — the absence of certain whole orders, as 

 of batrachians and of terrestrial mammals, notwithstanding the 



