350 GEOGRAPHICAL A^D GEOLOGICAL DISTRIBUTION. 



habiting the last two named island groups, their appearance there 

 is an accidental circumstance, depending upon the prevalence of 

 certain storm-winds by means of which an unlooked-for transport 

 has been effected ; and even in the case of the species found in the 

 Pacific islands, it is not exactly unlikely that the broad distribu- 

 tion has been brought about in a manner not necessarily indicating 

 sustained flight, or at a period when the physical configuration 

 of that portion of the earth's surface the relation of land to water 

 was different from what it is at the present day. For, as Mr. 

 Dobson suggestively points out, if the ' l Cheiroptera possess great 

 powers of dispersal, it is certain that quite nine-tenths of the spe- 

 cies avail themselves of them in a very limited degree indeed, 

 and it is significant that the distribution of the species is limited 

 by barriers similar to those which govern it in the case of other spe- 

 cies of mammals."* Thus, it is shown that out of a total of some- 

 what more than four hundred recognised species only about twenty- 

 five pass beyond the confines of the regions to which they properly 

 belong i. e., about ninety -five per cent, are characteristic regional 

 forms ; and of this number more than two-thirds belong to the pre- 

 eminently wandering family Vespertilionidse, which has by far the 

 broadest geographical distribution of any family of the order. If, 

 however, it be urged that this restriction is not so much due to any 

 inability on the part of the animals to migrate, but to considerations 

 connected with altered conditions of food and climate the influence 

 of which must be very marked we have the still more salient fact 

 presented that of the numerous flying-foxes (Pteropus) which in- 

 habit Madagascar and the Comoro Islands, not a single species is 

 found on the east coast of Africa, the narrow channel of one hun- 

 dred and eighty to two hundred miles which intervenes between 

 the continent and Great Comoro Island seemingly being sufficient 

 to form an effectual barrier to a westward migration. Still, in this 

 special instance we are, perhaps, not presented with a just criterion 

 of the actual powers of dispersion possessed by this class of ani- 

 mals, since the slow and laboured flight of the large flying-foxes 

 can scarcely be compared with that of the smaller insectivorous 

 species ; indeed, there is a striking similarity between the insectivo- 

 rous bat fauna of Africa and Madagascar. 



Instances of very broad specific distribution among the Cheirop- 

 * "Kept. Brit. Assoc.," 1878. 



