XIII EVOLUTION AND DISTRIBUTION OF ANIMALS 271 



area of many more. It is also important to remember 

 that the dominant or growing species and genera, which 

 are those having continuous areas, will be necessarily 

 more prominent, more numerous in species and individuals, 

 and therefore far better known ; while those in process of 

 extinction, and for that very reason having discontinuous 

 areas, will be less numerous, far less common, and in fact 

 often very rare, and therefore much less known. In many 

 cases, too, it will happen that the discontinuity is not 

 great as regards distance, and it will then not be noticed, 

 or will be imputed to want of knowledge, although it 

 may be quite as real as when half a continent lies between 

 the two species. 



It appears, therefore, that the discontinuity of many 

 genera and higher gi'oups, so far from being difficult of 

 explanation, is really one of the inevitable results of the 

 process of extinction which is always going on. The 

 peculiarity of the particular case we are considering is 

 that it is somewhat extreme in the fact of two species 

 only being left, occupying limited areas situated at the 

 opposite extremities of the immense Palsearctic region. 

 But this is not very extraordinary, because there are in 

 Western Europe and Japan a number of pairs of closely 

 allied species whose extinction in the intervening areas 

 would lead to an exactly similar phenomenon to that we 

 are considering. Such are the European and Japanese 

 jays, bullfinches, goldcrest warblers, and wrens, all of 

 which are closely allied to each other, while they are 

 separated by a wide area in Central Asia often occupied 

 by species which differ considerably from both. Should 

 either of these groups die out, we might expect that the 

 species inhabiting the comparatively desert and 

 inhospitable regions of Central Asia would succumb first, 

 while those living in the milder and more equable 

 climates of Western Europe and Japan would probably 

 linger on, the last of their race. It is very interesting to 

 note that in most cases of such widely separated but 

 closely allied species or groups there is a decided 

 similarity in the general physical conditions of the 

 countries they inhabit. The ally of the Spanish blue 



