CHAPTER XIII. 



EMBRYONIC ORGANS. METAMORPHOSIS AND ALTER- 

 NATION OF GENERATIONS. 



In our analysis of the development of an indi- 

 vidual organism from the germ tp maturity we 

 saw that two kinds of causes were at work in the 

 process. One set of causes comprises all those 

 conditions in the living matter of the germ which 

 are the result of the entire past experience of the 

 continuity of living matter of which the germ 

 formed a part. We have seen also how these condi- 

 tions have arisen as the result of long continued 

 repetition of the reactions of living matter to the 

 forces of the environment, — the effects gradually 

 accumulating by the power of nervous association 

 through the long course of ages and generations, 

 and forming what we have called the hereditary 

 impulse. The other set of causes controlling 

 individual development are those combinations of 

 external forces which act constantly and periodi- 

 cally upon the developing organism, supplying to 



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