CHAP. XIV.] THE NEOTROPICAL REGION. 35 



the sea, because we find existing species with an enormous 

 range. The ancestors of the AmhlyrhyncJii must have come as 

 early, probably, as the earliest birds ; and the same powers of 

 dispersal have spread them over every island. The two American 

 genera of lizards, and the tortoises, are perhaps later immigrants. 

 Latest of all were the snakes, which hardly difier from continental 

 forms ; but it is not at all improbable that these latter, as well as 

 the peculiar American mouse, have been early human importa- 

 tions. Snakes are continually found on board native canoes 

 whose cabins are thatched with palm leaves ; and a few cen- 

 turies would probably suffice to produce some modification of 

 a species completely isolated, under conditions widely different 

 from those of its native country. Land-shells, being so few and 

 small, and almost all modifications of one tj^Q, are a clear indi- 

 cation of how rare are the conditions which lead to their dispersal 

 over a wide extent of ocean ; since two or three individuals, ar- 

 riving on two or three occasions only during the whole period 

 of the existence of the islands, would suffice to account for the 

 present fauna. Insects have arrived much more frequently ; and 

 this is in accordance with their habits, their lower specific gravity, 

 their power of flight, and their capacity for resisting for some 

 time the effects of salt water. 



We learn, then, from the fauna of these islands, some very im- 

 portant facts. We are taught that tropical land-birds, unless 

 blown out of their usual course by storms, rarely or never venture 

 out to sea, or if they do so, can seldom pass safely over a distance 

 of 500 miles. The immigrants to the Galapagos can hardly have 

 averaged a bird in a thousand years. We learn, that of all reptiles 

 lizards alone have some tolerably effective mode of transmission 

 across the sea ; and this is probably by means of currents, and 

 in connection with floating vegetation. Yet their transmission 

 is a far rarer event than that of land-birds ; for, whereas three 

 female immigrants will account for the lizard population, at least 

 eight or ten ancestors are required for the birds. Land serpents 

 can pass over still more .rarely, as two such transmissions would 

 have sufiiced to stock the islands with their snakes ; and it is not 

 certain that either of these occurred without the aid of man. 



