226 AFFINITIES CONNECTING [Chap. XIV. 



go on increasing in size ; and they consequently sup- 

 plant many smaller and feebler groups. Thus we can 

 account for the fact that all organisms, recent and 

 extinct, are included under a few great orders, and 

 under still fewer classes. As showing how few the 

 higher groups are in number, and how widely they are 

 spread throughout the world, the fact is striking that 

 the discovery of Australia has not added an insect 

 belonging to a new class ; and that in the vegetable 

 kingdom, as I learn from Dr. Hooker, it has added only 

 two or three families of small size. 



In the chapter on Geological Succession I attempted 

 to show, on the principle of each group having generally 

 diverged mucli in character during the long-continued 

 process of modification, how it is that the more ancient 

 forms of life often present characters in some degree 

 intermediate between existing groups. As some few of 

 the old and intermediate forms have transmitted to 

 the present day descendants but little modified, these 

 constitute our so-called osculant or aberrant species. 

 The more aberrant any form is, the greater must be the 

 number of connecting forms which have been exter- 

 minated and utterly lost. And we have some evidence 

 of aberrant groups having suffered severely from ex- 

 tinction, for they are almost always represented by 

 extremely few species ; and such species as do occur 

 are generally very distinct from each other, which 

 again implies extinction. The genera Ornithorhynchus 

 and Lepidosiren, for example, would not have been less 

 aberrant had each been represented by a dozen species, 

 instead of as at present by a single one, or by two or 

 three. We can, I think, account for this fact only by 

 looking at aberrant groups as forms wliich have been 



