294 Heredity. 



life, for this very purpose. I believe that the gradual 

 establishment of this law of heredity is clue to the action 

 of natural selection; to the divergent specialization of the 

 two sexual elements; and to a physiological division of 

 labor, each step in the production of which has been 

 advantageous, and has therefore been perpetuated like 

 any other useful variation. 



According to this view, we must recognize in the law 

 of natural selection, not simply a great means of modifi- 

 cation, but the agency to which organic evolution is 

 almost exclusively due; but we must also believe that, 

 in the higher multicellular organisms it acts indirectly, 

 and is subordinate to another law, the law of heredity, 

 which itself owes existence to the law of natural se- 

 lection. 



Objection to the View that the Variation of any Part is 

 Caused by the Transmission of Gemmules, which owe 

 their Existence to the Action of Unfavorable Condi- 

 tions upon the Corresponding Part of the Parent. 



Mr. H. "W. Conn has called my attention to the fact 

 that in many cases it is difficult to see any connection 

 between the function of a new variation and a failure to 

 perform that function in the parent. He instances the 

 mimetic colors of insects, and the long neck of the 

 giraffe, and says that it is difficult to see how the action 

 of unfavorable conditions upon the parents could have 

 given rise to these variations. He says that if an insect 

 were dangerously conspicuous its unfavorable conditions 

 of life would not affect the cells to which its color is due, 

 in any especial way, but would lead to the destruction of 

 the entire animal. 



So, too, if a series of dry seasons should place the 

 giraffe under conditions of hardship, the individuals 



