INTERNAL FACTORS. 513 



cation in the proportions of the body. Increase in the size 

 of any organ implies a quantitative, and often a qualitative, 

 reaction on the blood; and thus alters the nutrition of all 

 other organs. Such physiological correlations are exemplified 

 in the many differences which accompany difference of sex. 

 That the minor sexual peculiarities are brought about by the 

 physiological actions and reactions, is shown both by the 

 fact that they are commonly but faintly marked until the 

 fundamentally distinctive organs are developed, and that 

 when the development of these is prevented, the minor sexual 

 peculiarities do not arise. No further proof is, I 



think, needed, that in any individual organism or its de- 

 scendants, a new external action must, besides the primary 

 internal change which it works, work many secondary 

 changes, as well as tertiary, changes still more multiplied. 

 That tendency towards greater heterogeneity which is given 

 to an organism by disturbing its environment, is helped by 

 the tendency which every modification has to produce other 

 modifications modifications which must become more nu- 

 merous in proportion as the organism becomes more com- 

 plex. Lastly, among the indirect and involved manifesta- 

 tions of this tendency, we must not omit the innumerable 

 small irregularities of structure which result from the cross- 

 ing of dissimilarly-modified individuals. It was shown 

 (89, 90) that what are called "spontaneous variations," 

 are interpretable as results of miscellaneously compounding 

 the changes wrought in different lines of ancestors by differ- 

 ent conditions of life. These still more complex and multi- 

 tudinous effects so produced, are further illustrations of the 

 multiplication of effects. 



Equally in the aggregate of individuals constituting a 

 species, does multiplication of effects become the continual 

 cause of increasing multiformity. The lapse of a species into 

 divergent varieties, initiates fresh combinations of forces 

 tending to work further divergences. The new varieties 

 compete with the parent species in new ways; and so add 



