GEOGRAPHIC DISTRIBUTION 317 



is believed to have been increased by a land connection between them 

 across the Bering Strait and the adjoining Arctic Ocean, which would 

 have permitted migration between them up to (geologically) compara- 

 tively recent times. 



In all these instances the argument is that the similar animals have 

 had more recent common ancestors, and there has been less time to 

 migrate far away. The effect of a barrier, for example, the Rocky 

 Mountains helping to keep eastern and western mammals apart, is 

 merely to push back the time of the common ancestors of the less similar 

 types, and so make their dissimilarities greater. 



Normal Migration. — So important in the explanation of these pecu- 

 liarities of distribution are the abilities of the members of species to 

 spread, and the time they have had at their disposal to attain their 

 present locations, that the means by which they have become dispersed 

 should be examined. By far the most important method is what may 

 be called their normal migration. This is best seen in freely moving 

 terrestrial forms. The individual seeks food or shelter, avoids enemies, 

 seeks a mate. How rapidly it moves depends on its powers of loco- 

 motion. Whether it goes alone or in flocks or herds depends on little 

 understood psychology. These activities lead inevitably to the occu- 

 pation of more territory, unless barriers forbid, and by a young species 

 barriers are not as a rule reached very soon. 



This spread by normal migration is ordinarily very gradual. Under 

 special circumstances, however, it may be greatly accelerated for a time. 

 The potato beetle, Leptinotarsa decemlineata, was long restricted to the 

 Rocky Mountains and the plains east, as far as western Kansas and 

 Nebraska. It could go no farther because its natural food, a wild species 

 of Solanum, did not exist east of that area. As the western part of the 

 Mississippi valley became settled, the range of the cultivated potato 

 (Solanum tuberosum) extended farther and farther west, until between 

 1845 and 1850 it reached the range of Leptinotarsa. The beetle found 

 the new Solanum a suitable food, so the eastern barrier was removed. 

 In about 20 years it had reached the Atlantic seaboard, where it stopped 

 until about 1918. Presumably in troop movements and shipment of 

 food supplies in the war, the beetle was carried to Europe, where it has 

 since existed despite efforts to eradicate it. 



How effective normal migration may be in spreading species is indi- 

 cated by some computations. For the slow-moving earthworms, Gado iv 

 calculates that if one pair produces enough offspring to occupy one square 

 yard of soil in one year, their descendants in the time since the Ice Age 

 (perhaps 30,000 years) would have choked the earth. Again, if a human 

 family moved, gypsy fashion, only one day a week, and not more than 

 three miles, then it would wander 156 miles each year; and the Mongo- 



