LAWS OF DISTRIBUTION. 169 



have arisen in some definite and usually restricted 

 locality, either slowly or suddenly by a change in 

 the conditions of its ancestors. This locality must 

 then be the centre of distribution. It may subse- 

 quently extend its range so as to cover eventually 

 a large area. But evidently this area must be, at 

 first, a continuous one, such as may be accounted 

 for by migration, for we cannot expect the same 

 species to arise in two localities independently. If, 

 therefore, we find the same species in two widely 

 separated localities, evolution must assume that it 

 formerly also inhabited the intermediate tract of 

 country, and that, owing to causes which can seldom 

 be explained, it has disappeared from the inter- 

 mediate regions, while it still remains at the ex- 

 tremes. If, for instance, the central States of our 

 country should cease to cultivate the potato, the 

 potato-beetle would disappear from this region, while 

 still continuing to live in the West and the East ; and 

 we should thus have a case of a single species in- 

 habiting two separated localities. The same prin- 

 ciples would be true in regard to the distribution of 

 genera, although here they would be less exact, 

 since the greater age of the genera would give 

 greater chance for wide distribution and extinction ; 

 but we should still look for the distribution of the 

 species of a genera corresponding to their relation 

 to each other. In the case of families, still greater 

 range of variation would be expected, since they are 

 doubtless very old, and have had time during their 

 existence to migrate to all parts of the globe, and to 

 have become extinct in many localities. It will cause 



