TRANSLATOR'S INTRODUCTION xiii 



doubling divisions, and that there is evidence that 

 divisions always are doubling divisions. He thinks, 

 in fact, that when a portion of germplasm divides, 

 the daughter- cells receive portions of germplasm 

 exactly alike and exactly like the original portion 

 in the parent-cell. The cells, indeed, become 

 different from each other as the organism grows, 

 some becoming muscle-cells, others nerve-cells, 

 others digestive-cells, and so forth. Weismann 

 thinks that the differences occur because, in the 

 disintegration of the germplasm - microcosms, 

 according to a prearranged plan, only the deter- 

 minants for nerve-cells are marshalled into nerve- 

 cells, only those for muscle-cells into muscle-cells, 

 and so forth. The development is an evolution, an 

 unfolding or unwrapping of little rudiments that 

 lie in the germplasm. Hertwig insists that every 

 cell receives the same kind of germplasm, but that, 

 according to the situations in which they come to 

 lie, different characters are impressed upon them. 

 The development is an epigenesis, or impressing on 

 identical material of different characters by dif- 

 ferent surrounding forces. His second line of 

 argument against Weismann leads to a similar 

 conclusion. A large number of the characters that 

 arise in an organism during its development are 

 due to the combination of many cells. They 

 cannot come into existence until the multiplication 

 of cells has made their existence possible, and he 

 thinks, therefore, that they cannot have rudiments 

 inside a single cell as their determining cause. 

 It is no part of my present purpose to insist, 



