Chap. VI. AFFINITIES AND GENEALOGY. 211 



been known occasionally in man and other mammals to 

 become well developed, and to yield a fair supply of 

 milk. Now if we suppose that during a former pro- 

 longed period male mammals aided the females in 

 nursing their offspring, and that afterwards from some 

 cause, as from a smaller number of young being pro- 

 duced, the males ceased giving this aid, disuse of the 

 organs during maturity would lead to their becoming 

 inactive ; and from two well-known principles of in- 

 heritance this state of inactivity would probably be 

 transmitted to the males at the corresponding age of 

 maturity. But at all earlier ages these organs would 

 be left unaffected, so that they would be equally well 

 developed in the young of both sexes. 



Conclusion. — The best definition of advancement or 

 progress in the organic scale ever given, is that by 

 Von Baer; and this rests on the amount of differ- 

 entiation and specialisation of the several parts of 

 the same being, when arrived, as I should be inclined 

 to add, at maturity. Now as organisms have become 

 slowly adapted by means of natural selection for 

 diversified lines of life, their parts w r ill have become, 

 from the advantage gained by the division of physio- 

 logical labour, more and more differentiated and spe- 

 cialised for various functions. 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 will still retain 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 

 organisation on the whole has advanced throughout the 

 world by slow and interrupted steps. In the great 



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