1 64 The Descent of Man. Part [. 



organs would be left unaffected, so that they would he almost 

 aqually well developed in the young of both sexes. 



Condusirin. — Von Baer has defined adTanoement or progress in 

 the organic scale better than any one else, as resting on the 

 amount of differentiation and specialisation of the several parts 

 of a being, — when arrived at maturity, as I should be inclined ta 

 add. Now as organisms have become slowly adapted to diver- 

 sified lines of life by means of natural selection, their parts will 

 have become more and more differentiated and specialised for 

 various functions, from the advantage gained by the division o< 

 physiological labour. The same part appears often to have been 

 modified first for one purpose, and then long afterwards for 

 some other and quite distinct purpose ; and thus all the parts 

 are rendered more and more complex. But each organism still 

 retains the general type of structure of the progenitor from 

 which it was aboriginally derived. In accordance with this 

 view it seems, if we turn to geological evidence, that organisa- 

 tion on the whole has advanced throughout the world by slow 

 and interrupted steps. In the great kingdom of the Vertebrata 

 it has culminated in man. It must not, however, be supposed 

 that groups of organic beings are always supplanted, and dis- 

 appear as soon as they have given birth to other and more 

 perfect groups. The latter, though victorious over their pre- 

 decessors, may not have become better adapted for all places in 

 the economy of nature. Some old forms appear to have survived 

 from inhabiting protected sites, where they have not been 

 exposed to very severe competition ; and these often aid us in 

 constructing our genealogies, by giving us a fair idea of former 

 and lost populations. But we must not fa'l into the error of 

 looking at the existing members of any lowly-organised group as 

 perfect representatives of their ancient predecessors. 



The most ancient progenitors in the kingdom of the Vertebrata, 

 at which we are able to obtain an obscure glance, apparently 

 consisted of a group of marine animals,'" resembling the larvse of 

 existing Ascidians. These animals probably gave rise to a 



3' The inhabitants of the sea- ditious for many generations, can 



shore must be greatly affected by hardly fail to run their cour.se in 



the tides ; animals living either regular weekly periods. Now it is a 



about the mean high-water mark, mysterious fact that in the higher 



or about the menn low-water mark, and now terrestrial Vertebrata, as 



pass through i complete cycle of well as in other classes, many nor- 



tidal changes in a fortnight. Con- mal and abnormal processes have 



sequently, their food supply will one or more whole weeks as their 



undergo marked changes week by periods ; this would be rendered 



week. The vital functions of sucn intelligible if the Vertebrata are d.e- 



kuimals, liviog under those con- Bcended from an animal allied tfl 



