Heredity and Natural Selection. 293 



ism, placed under new conditions, becomes modified to 

 meet the change in its environment, the existence of the 

 internal change is caused by the external change, while 

 its precise character is determined by other factors, 

 chiefly by the hereditary characteristics of the corre- 

 sponding jmrt, in both parents. 



As long as the harmony which has been gradually 

 established, by natural selection, between any particular 

 cell and its conditions of life, remains undisturbed, this 

 cell will continue to perform its function as a part of 

 the body, and will have little tendency to give rise to 

 gemmules. When through any change, either in the 

 conditions of life external to the organism, or in other 

 parts of the body, this cell comes to be placed in circum- 

 stances which are unfavorable to the performance of its 

 function, it will exert the tendency to throw off gem- 

 mules; for each cell being, in a morphological sense, an 

 independent organism, possesses this power, by inherit- 

 ance, although natural selection has gradually acted, 

 during the past history of the evolution of life, to pre- 

 vent the useless manifestation of the tendency, as long 

 as surrounding conditions are favorable and no change 

 is needed. 



These gemmules, when transmitted to the ^gg, by 

 impregnation, will, by sexual union with the correspond- 

 ing parts of the Qgg^ cause variation in the homologous 

 cells of the offspring, and will thus produce a congenital 

 hereditary change at the very time when, and in the 

 very part where, such change is needed. 



Instead of being purely fortuitous on the one hand, 

 or due on the other hand to the direct modifying influ- 

 ence of external conditions, congenital variations are 

 due to the manifestation of a general law, which has 

 gradually become established, during the evolution of 



