176 _ Heredity and Environment 



differential cause in the germ cells, but this cause may not be a 

 peculiar vital corpuscle nor even a peculiar atom or molecule but 

 it must at least be some particular combination of atoms or mole- 

 cules. It is practically certain that this differential cause of each 

 inherited character is associated with some material substance 

 which occupies some place in the germ cells. 



Location of Inheritance Factors. If it is asked whether there 

 are particular structures in germ cells which correspond to par- 

 ticular inheritance factors it must be admitted that we have no 

 certain knowledge on this subject and that opinions differ greatly 

 with respect to it. On the one hand it is maintained that the 

 entire germ is concerned in the development of every' character, 

 and on the other hand that the differential cause of any character 

 may be located in some differentiated structure or function of the 

 germ. These views are not mutually exclusive and it may well 

 be that both are true. We know that germ cells are composed of 

 many parts which differ from one another in both structure and 

 function, and it is highly probable that there are enough of these 

 parts to provide a locus for every inheritance factor. 



There was a time when the cell was the Ultima Thule of biologi- 

 cal analysis and when the contents of cells were supposed to be 

 "perfectly homogeneous, diaphanous, structureless slime." Then 

 the nucleus was discovered within the cell, then the chromosomes 

 within the nucleus, then the chromomeres within the chromosomes, 

 and there is no reason to suppose that organization ceases with the 

 powers of our present microscopes. With every improvement of 

 the microscope and of microscopical technique, structures have 

 been found in cells which were undreamed of before, and it is 

 not probable that the end has been reached in this regard. We 

 know that cells contain nuclei and chromosomes and chromo- 

 meres, centrosomes and plastosomes and microsomes, and we 

 know that some of these parts differ in function as well as struc- 

 ture. And there is no reason to doubt that if we had sufficiently 

 powerful microscopes we should find still smaller and smaller 

 units until we came at last to molecules and atoms. 



