206 Life and Death, Heredity and Evolution 



It is natural that an alteration of a primary hereditary 

 character in the germ cell of a higher organism should, when 

 the long development from that germ cell is completed, pro- 

 duce a much more extensive and more marked effect than in 

 a Protozoan. For in the latter it is the same cell that is 

 altered which forms the adult, with relatively little develop- 

 ment, and with no intervening multiplication of cells. But 

 in the higher organism the altered germ cell goes through a 

 great number of cell divisions, accompanied by continuous 

 interactions of the different substances in the nuclei, result- 

 ing in an enormous increase in differentiation, in numbers of 

 cells, and in bulk. All these cells, and this entire bulk, may 

 therefore show the results of the slight original change. If 

 some substance necessary for the production of the red eye 

 color of the fruit-fly were omitted from the germ cell, it is 

 probable that the change in the germ cell would itself be 

 so slight that it could not be detected by any physical or 

 chemical tests at present available. It is little more than 

 changes corresponding to this that we may expect to find 

 in the organisms made up of but one cell. 



But must a hereditary change in the adult characters of 

 a higher organism necessarily be such a saltation; a change 

 of large extent? On this point the state of knowledge has 

 greatly changed with the thorough studies made in recent 

 years, although the change has as yet been little appreciated 

 outside the field of specialists working on these matters. 

 We shall attempt to give a brief sketch of the position of 

 this question in higher organisms, for comparison with what 

 we have seen in the lower ones. 



It has been found that in many higher organisms it is 

 possible through long continued breeding with careful selec- 

 tion of the parents, to gradually cause a change in the hered- 

 itary characteristics shown by the stock. Often this change 



