It is possible to see, at any rate, three causes which are responsible for a large number of such cases. In the first place, a given species of animal must have originated at a certain spot; its multiplication in individuals must always be a slow matter, since enemies, and untoward events generally, would conspire to check the natural multiplication by geometrical progression. A long time might therefore elapse before the species greatly extended its range. A restricted distribution may therefore, in some cases, mean a modern race. In the second place, there are definite physical barriers which check the migration of species. The terrestrial Mammalia cannot cross wide arms of the sea; that they can and do swim for considerable distances has been proved in several instances; but, as has been pointed out, it is unlikely that a purely terrestrial mammal would voluntarily swim out into an unknown sea. And then if it did, and successfully reached the opposite side, nothing would happen unless it were a pregnant female; or, if not pregnant, till a male swam very soon afterwards in exactly the same direction. Many travellers have told of floating islands, formed of torn-up trees and brushwood, which have been seen at the mouths of large rivers, with animal passengers upon them. These are, however, so much at the mercy of currents and storms, {81} that but little reliance can be placed on them as a means of transit; besides, here again, two individuals, or a pregnant female, would be required to effect a settlement on a foreign shore. The existence of oceanic islands is often urged as a proof of this inability to cross tracts of sea; even those which are comparatively near an extensive continent, such as, for example, Fernando Noronha in the Atlantic, are destitute of mammals (except, indeed, the ubiquitous Mouse, which is believed to have been carried there, often in company with the equally widely-spread Rat, in ships). This argument, however, is not so conclusive as might appear; it doubtless is in the case of far-distant islands. But the size of the islands has to be taken into account. For there are islands, such as the Galapagos, or, to take a less contested instance, some of the islands of the Malagasy Archipelago, undoubtedly continental, which have an exceedingly reduced number of mammals. An area of a certain size seems to be a necessity.