162 EVOLUTION AND GENETICS 



their relation to development and inheritance, made 

 great advances and cleared up many obscure ques- 

 tions. These observations were carried out in com- 

 plete independence of the speculations concerning 

 heredity that had gone before ; and the outcome has 

 furnished a starting point for further interpreta- 

 tions that have led in our own time to far reaching 

 discoveries. It is not possible to give here even a 

 summary of the evidence, because its understanding 

 requires familiarity with microscopic observations 

 covering a very wide and unfamiliar field. But, in 

 general, I may state that the work has led to the 

 conclusion that the properties of the reproductive 

 cells which are responsible for the characters of the 

 body, are inherent in these cells ; and that the trans- 

 mission of these properties is independent of the 

 body-cells, and calls for no interference from them. 

 This is summed up in the phrase "the isolation of the 

 germ-plasm." The principal idea that this familiar 

 phrase is intended to convey is exactly the opposite 

 of that implied in the inheritance of acquired char- 

 acters. The individual starts as an ^^^ which is itself 

 a cell. The ^^^ divides and ^^roduces a vast number 

 of cells essentially like itself. ]Most of these cells be- 

 come changed, as development proceeds, into the 

 tissues and organs of the body, but a few of them 

 remain as the reproductive cells of the individual in 

 which they live. Here they multiply to become each 

 in turn the beginning of a new individual with its 



