INTERNAL FACTORS. 425 



and muscular systems. Besides this multiplica- 



tion of mechanical effects, there is a multiplication of 

 physiological effects. The vascular apparatus is modified 

 throughout its whole structure, by each considerable modifi- 

 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 so alters the nutrition of all other 

 organs. Such physiological correlations are exemplified in the 

 many differences that accompany difference of sex. That the 

 minor sexual peculiarities are brought about by the physio- 

 logical 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 de- 

 velopment 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 descendants, a new 

 external action must, besides the primary internal change 

 which it works, work sundry secondary changes, as well as 

 tertiary changes still more multiplied. That tendency to- 

 wards greater heterogeneity which is given to an organ- 

 ism by disturbing its environment, is helped by the tendency 

 which every modification has to produce other modifications 

 - — modifications which must become more numerous in pro- 

 portion as the organism becomes more complex. And 

 then, lastly, among the indirect and involved manifestations 

 of this tendency, we must not omit the iimumerable small 

 irregularities of structure that result from the crossing of 

 dissimilarly-modified individuals. It was shown (§§ 89, 90) 

 that what are called "spontaneous variations," are inter- 

 pretable as results of miscellaneously compounding the 

 changes wrought in different lines of ancestors by different 

 conditions of life. These still more complex and multi- 

 tudinous effects so produced, are thus further illustrations of 

 the multiplication of effects. 



Equally in the aggregate of individuals constituting a 

 species, does multiplication of effects become the continual 

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