26 WIDE RANGE OF ANCIENT FORMS. [CH. I 



within recent times has brought about the great diver- 

 gence which is now seen between the two sections of the 

 Primates, the new world Platyrrhines and the old world 

 Catarrhines. It has been already pointed out in dealing 

 with the distribution of the archaic Peripatus that the 

 species of different parts of the world form natural 

 assemblages separable from those of other parts of the 

 world by definite anatomical characters. This is the case 

 too with many other genera and families of animals. We 

 invariably find that when a group, which Palaeontology 

 or in the absence of direct evidence from fossils, other 

 considerations derived from anatomy or embryology 

 proves to be an ancient group, has a tolerably wide and 

 discontinuous distribution, marked differences in structure 

 distinguish its representatives in different parts of the 

 world. This is more marked still in the case of a group 

 which has but limited powers of dispersal. We shall now 

 illustrate the connection between distribution and ana- 

 tomical structure by a few examples, which are of course 

 few among many ; in other pages other instances have 

 been or will be treated of and reference may be made to 

 those places for further illustration of the general fact. 



Distribution of the Gallinaceous birds. 



The Gallinaceous birds (Alectoromorphse of Huxley) 

 offer an exceedingly instructive example of the connection 

 between anatomical structure and geographical distri- 

 bution. There can be no doubt that this group is a 

 natural one. It is divisible into the following seven 



