150 ON VARIABILITY AND ADAPTATION 



and directive body in the cell — that it is the nuclei which undergo 

 conjugation — and that, in short, we have to recognize that the 

 idioplasm of the individual is contained in the nucleus. It is 

 by no means necessary to conceive that the whole of the nuclear 

 material is idioplasm, or that the whole of the idioplasm enters 

 into chemical combination with the molecules derived from 

 the conjugating germ cell, molecule for molecule. I mention 

 this in passing to indicate that it is not necessary to assume 

 that when polar bodies are extruded, the material forming them 

 is identical with the idioplasm of the remaining nuclear material. 

 The difficulties in explaining polar bodies and the reduction of 

 the nuclear substance are no greater by this than by Weismann's 

 theory. The exigencies of time demand that I should not enter 

 in detail into the consideration of these subjects. Rather I 

 must give the broad outlines of the theory and pass on to consider 

 inheritance in multicellular organisms. 



Inheritance in Multicellular Forms. — Every multicellular 

 organism arises from a single cell, the fertilized ovum, itself in 

 all sexually produced forms the result of fusion of a male and 

 female cell, of the ovum and spermatozoon. Studying the 

 method whereby this one cell gives rise to the adult multi- 

 cellular organism, we see that by successive acts of division this 

 one cell gives rise to all the cells of the body. In the course of 

 this process its nucleus divides and redivides, and this in such 

 a way that at each division like portions of the nuclear material 

 pass into each daughter cell. This regular distribution of nuclear 

 material affords a sound ground for believing that there is an 

 equally regular and uniform distribution of idioplasm into each 

 cell. Now I have already pointed out the value of Driesch's 

 observations in overthrowing Weismann's contention that there 

 must be a qualitative difference in the idioplasm distributed to 

 the daughter cells. We have absolutely no ground for believing 

 in any such qualitative difference ; on the other hand, it is a 

 legitimate inference that the idioplasm is modified by its environ- 

 ment. To take the simplest example — a multicellular organism 

 composed of a spherical mass of cells. Those cells which come 

 to form the peripheral layer of the individual are subjected to 

 a series of reactions quite different from the series telling upon 

 the centrally situated cells, and it requires no stretch of the 

 imagination to predict that in the process of assimilation and 



