SEGREGATION. 485 



in such way as effectually to prevent it from being bent, 

 laterally or vertically, in the slightest degree. This bony 

 box, which must have weighed several hundred-weight, was 

 supported on the spinous processes of the vertebrae, and on 

 the adjacent bones of the pelvic and thoracic arches. And 

 the significant fact now to be noted, is, that here, where the 

 trunk vertebrae were together exposed to the pressure of this 

 heavy dermal armour, at the same time that, by its rigidity, 

 they were preserved from all relative movements, the entire 

 series of them were united into one solid, continuous bone. 

 The formation and maintenance of a species, considered 

 as an assemblage of similar organisms, is interpretable in 

 an analogous way. We have already seen that in so far as 

 the members of a species are subject to different sets of inci- 

 dent forces, they are differentiated, or divided into varieties. 

 And here it remains to add that in so far as they are subject 

 to like sets of incident forces, they are segregated, or reduced 

 to, and kept in, the state of a uniform aggregate. For by the 

 process of " natural selection," there is a continual purifica- 

 tion of each species from those individuals which depart 

 from the common type in ways that unfit them for the con- 

 ditions of their existence. Consequently, there is a contin- 

 ual leaving behind of those individuals which are in all re- 

 spects fit for the conditions of their existence ; and are there- 

 fore very nearly alike. The circumstances to which any 

 species is exposed, being, as we before saw, an involved com- 

 ^jnation of incident forces; and the members of the species 

 ha\lng mixed with them some that differ more than usual 

 from the average structure required for meeting these 

 forces; it results that these forces are constantly separating 

 such divergent individuals from the rest, and so preserving 

 the uniformity of the rest keeping up its integrity as a spe- 

 cies. Just as the changing autumn leaves are picked out by 

 the wind from among the green ones around them, or just as, 

 to use Prof. Huxley's simile, the smaller fragments pass 

 through the sieve while the larger are kept back; so, the 



