380 



THE AMERICAN NATURALIST 



[Vol. L 



persal, and that what is now the center of greatest development 

 is or will be a new center of dispersal provided there are means 

 of dispersal. There seems to be a genetic relationship between 

 these three centers and, of course, at the beginning the center 

 of origin is also the other two. 



Little need be said here about means of dispersal except to 

 point out what Matthew also emphasized, namely, that time is 

 long and luck is real. Those of us who have been brought up on 

 the doctrine of evolution by selection of "chance" variations 

 should have a whole-hearted respect for that sometimes abused 

 word. Those who believe there lias been time enough for alert 

 Nature to seize upon enough chances to differentiate 400,000 spe- 

 cies of insects. Tor example, need not strain unduly in swallowing 

 the in it ion that a very small proportion of these have been able 

 to get across relatively short stretches of water without a bridge. 

 Those who believe that Nature not only seized upon but made 

 opportunities for the differentiation of species should have no 

 trouble in discovering easier ways for her to help her creatures 

 spread their range than by raising up long narrow portions of 

 the ocean bed for a certain few to cross dry shod and then sink- 

 ing it to the discomfiture of those which are not of the elect. 



Insects probably get about as easily as any creatures because 

 most insects fly or may be blown long distances and, further- 

 more, the majority have at least two distinct stages in their life 

 cycle during which they remain inert and without the necessity 

 of feeding. If mammals can reach islands not connected by 

 bridges, surely insects of many kinds can. Matthew places great 

 stress on natural rafts as a means of transporting mammalian 

 fauna. After giving briefly a "series of facts and assumptions 

 [which] may serve to give some idea of the degree of probability 

 that attaches to the hypothesis of over-sea transportation to ac- 

 count for the population of oceanic islands" he says: 



If then we allow that ten such eases of natural rafts far out at sea 

 have been reported, we may concede that 1,000 have probably occurred 

 in three centuries and 30.0iM).nno during the Cenozoic. Of these rafts, 



