We may thus distinguish between the natural or active and the accidental or passive means of distribution of animals. The active mode of dispersal again may be only migratory, as in most animals, or periodic and migratory, as in some birds and fishes. It is of course the tendency of every species to spread in all directions from its original home, provided it does not encounter obstacles, such as want of food, unsuitability of climate or soil, or barriers such as mountains, rivers, or the sea. Birds might be thought to be little interfered with by any of these barriers, but, as Dr. Wallace has shown, they are almost as much affected by them in their distribution as mammals are. This then is the ordinary migratory distribution. Periodic distribution obtains with migratory birds and fishes. The annual flight of swallows to their northern summer residence comes under the heading of periodic migration or distribution, but apart from this, the swallow must seek to extend its range by the ordinary method, like every other animal. Similarly, the herring migrates periodically into shallow water to spawn, only to return again to its deeper home, where, as its numbers increase, there must be a tendency to spread. We have in these cases, therefore, both a periodic and an ordinary movement of migration. Now, in studying the composition of a fauna, and especially its origin, it is of the utmost importance to be able to determine approximately the percentage of accidental arrivals and of the ordinary migrants--that is to say, of those which have reached the country owing to accidental distribution, and of animals which have adopted the usual course of migration. It is of all the more import to review this subject of accidental, or, as Darwin called it, "the occasional means" of distribution, as both he and Dr. Wallace have, I venture to think, somewhat over-estimated its significance. No one doubts that accidental transportal takes place, but the question is whether the accidentally transported animals arrive living and reach a spot where suitable food is procurable, and whether they are able to propagate their own species in the new locality. For it must be clear to anybody that the accidental transportal of a beetle or of a snail to a new country cannot affect its fauna or add one permanent member to it unless all these conditions are fulfilled. As a matter of fact, only exceedingly few instances are on record of man having witnessed, for example, the accidental transportal across the sea to an island of a live animal.