546 



SCIENCE. 



[N. S. Vol. XXII. No. 566. 



quite side by side with the old. It is 

 simply next to it, geographically or geolog- 

 ically speaking, and the degree of distinc- 

 tion almost always bears a relation to the 

 importance or the permanence of the bar- 

 rier separating the supposed new stock 

 from the parent stock. 



A flood of light may be thrown on the 

 theoretical problem of the origin of species 

 by the study of the probable origin of spe- 

 cies with which we may be familiar, or of 

 which the actual history or the actual rami- 

 fications may in some degree be traced. 



In such cases, one of the first questions 

 naturally asked is this : Where did the spe- 

 cies come from? Migration forms a large 

 part of the history of any species or group 

 of forms. The fauna of any given region 

 is made up of the various species of animals 

 living naturally within its borders. The 

 flora of a region is made up of the plants 

 which grow naturally within its borders. 

 Of all these, animals and plants, the in- 

 habitants of most regions are apparently 

 largely migrants from some other region. 

 Some of them have entered the region in 

 question before acquiring their present 

 specific characters ; others come after hav- 

 ing done so. AVhich of these conditions 

 apply to any individual case can generally 

 be ascertained by the comparison of individ- 

 uals along the supposed route of migration. 

 Thus the Tahoe trout is clearly a migrant 

 from the Columbia River, its separation 

 apparently dating from the time when the 

 former lake basin of Nevada (Lake Lahon- 

 tan) found its drainage in the Columbia. 

 We know that the present specific char- 

 acters of this trout were assumed within 

 its present range, because these traits are 

 not found in any other trout along the sup- 

 posed line of its migrations. On the other 

 hand, we feel certain that the salmon of 

 New England originated as a species in 

 Europe. In extending its range from its 

 primitive home, it has not undergone any 



material change or acquired any distinctive 

 characters, while both in Europe and in 

 America it has more or less modified land- 

 locked variants. 



The region from which a group of ani- 

 mals is supposed to be derived is spoken of 

 as its center of dispersion. 



Thus the various forms of trout, orig- 

 inally of marine origin, seem to have di- 

 verged from the Siberian region, eastward 

 and westward. This region is, therefore, 

 the center of dispersion of trout. In like 

 manner the tanagers have their center of 

 dispersion in South America ; the humming 

 birds also. The lemurs have their center 

 of dispersion in Madagascar, and the cat 

 tribe in tropical Africa. 



In general, although not always, the 

 original home of any group of animals will 

 show more varied forms than those in any 

 other region. There may be some repre- 

 sentatives more highly specialized, more 

 primitive forms are likely to be preserved, 

 more degenerate forms may be developed, 

 and if the type is represented by fossils, 

 these also may exhibit a larger range of 

 forms and characters than will be found in 

 regions occupied through later migrations. 



It is evident that the nature of any 

 fauna bears an immediate relation to the 

 barriers, geographical or climatic, that sur- 

 round it. Whenever the free movement of 

 a species is possible, this involving the free 

 interbreeding of its members, the characters 

 of a species remain substantially uniform. 



Whenever free movement and interbreed- 

 ing is checked, the character of the species 

 itself is altered. This is the meaning of 

 Coues's pregnant phrase: 'Migration holds 

 species true ; localization lets them slip. ' 

 In other words, free interbreeding swamps 

 the incipient lines of variation, and this in 

 almost every case. On the other hand, a 

 barrier of any sort brings a certain group 

 of individuals together. These are sub- 

 jected to a selection different from that 



