1915^ Outline on the Theory of Descent 23 



The ovary is removed from a young female, white in color, 

 and that from a black animal substituted. The animal 

 operated upon is then mated with a male pure white in color. 

 All the offspring were absolutely black. The black ovary 

 although transplanted to a white animal produced young 

 which were colored exactly the same as would have been the 

 case had the ovary remained in the body of the black animal 

 from which it was taken. This is in accord with Mendelian 

 principles — black dominating white. In other words the 

 body tissues of the foster-mother had no effect whatever, upon 

 the color of the developing embryo. Castle, as well as others, 

 has regarded such experiments as lending great support to 

 the view that the environment produces no heritable effects. 

 The lifework of August Weisman, the great German biolo- 

 gist and philosopher, who died last fall, was in a sense a 

 continuous and formidable onslaught upon Lamarck's hypo- 

 thesis. Weisman founded his attack upon the fact that the 

 germ substance is very generally early set apart in the de- 

 velopment of each animal from egg to adult. Once so set 

 apart, he contended, the germ cells could not be influenced 

 by the tissue cells, a thing must happen if a character im- 

 pressed upon the body cells is to become hereditary. This 

 has now become to be pretty generally the dominant view 

 with geneticists and embryologists. The Weismannian 

 contention, however, is not so formidable as it once seemed.. 

 It is a well known fact that the germ cells exercise a profound 

 influence on the body cells. Is it too wild a flight to suppose 

 that there might be a reverse influence ? At any rate such a 

 view becomes somewhat more tenable in the light of recent re- 

 searches on the ductless glands, which have their effect in 

 distant parts of the body by means of substances secreted into 

 the blood. These substances are called harmones, and it is 

 coming to believed that they are of very general occurrence 

 and very great importance. 



The fact upon which Weisman based his criticisms seems 

 to me to furnish a basis for the more or less evident line of 

 cleavage between zoologists and botanists on this question. 



