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 
JlJ dk. 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 
Ca use, 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 
tractive; 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 
he left unaffected, so that they would be equally well 
developed in the young of both sexes. 
Conclusion. — The best definition of advancement or 
pr°g ress in the organic scale ever given, is that by 
' °u Baer; and this rests on the amount of differ- 
entiation and specialisation of the several parts of 
same being, when arrived, as I should bo inclined 
0 add, at maturity. Now as organisms have become 
alowly adapted by means of natural selection for 
diversified lines of life, their parts will have become, 
from the advantage gained by the division of physio- 
iogical labour, more and more differentiated and spe- 
cialised for various functions. The same part appears 
°ften to have been modified first for one purpose, and 
t]l en long afterwards for some other and quite distinct 
Purpose ; and thus all the parts are rendered more and 
^ore complex. But each organism will still retain the 
general type of structure of the progenitor from which 
ll: ' w as aboriginally derived. In accordance with this 
'lew it seems, if we turn to geological evidence, that 
Ol ganisation on the whole has advanced throughout the 
" 01 ld by slow and interrupted steps. In the great 
