2i8 DARWINISM TO-DAY. 



cells of the body of the parent? And that is precisely the charac- 

 teristic assumption, dressed up in an ingenious variety of form, 

 which charac. crises the theories of life-units most favoured at 

 present : such theories are those of Darwin, Galton, Brooks, Nageli, 

 de Vries, Hertwig, and Weismann. In some of these, interesting 

 attempts are introduced to connect the assumed structure with 

 the actually observed finer structure of the nuclear protoplasm, by 

 introducing combinations of the fundamental units, in one or two, 

 or even three, successive degrees until an aggregation is reached 

 which corresponds with those microscopic structures, the chromo- 

 somes, or chromatin granules or threads, which are actually visible 

 to the microscope-aided eye. The most recent one of the theories 

 of this general type is that of Weismann's biophors and determi- 

 nants structure of the germ-plasm, already explained in connection 

 with the presentation of his theory of germinal selection (see pp. 

 193 ff.). As other examples we may note especially Darwin's, 

 called the theory of the pangenesis of gemmules ; and Nageli's, 

 called the theory of micellae and idioplasm. 



Darwin's gemmules are extremely minute particles, which are 

 formed in all the various cells of the body and are capable of repro- 



Darwin's ducing themselves rapidly and in great numbers by 



theory. repeated division, and which, by virtue of their minute 



size and an innate activity due to a sort of affinity or attraction exist- 

 ing between them and other substances, move about freely in the body, 

 penetrating any membranes, and arranging themselves with a deli- 

 cate precision just where they are most needed. When a gemmule 

 enters an undifferentiated or developing cell as yet containing no 

 other gemmules, it controls the development of that cell so that it 

 becomes a cell of the type from which the gemmule had birth, each 

 gemmule representing thus exactly the characteristics and the type 

 of its mother cell. Thanks to the delicate and precise adjustment 

 of affinities, migrating gemmules only enter those cells which they 

 really should enter in order that a normal development of all the 

 cells of the body should go on. But those few cells of the body 

 which are destined to become germ-cells, that is the spermatozoids 

 and eggs in animals, the pollen grains and ovules in plants, receive 

 during their formation gemmules from all the other cells of the 

 body. Not only from all the cells of the fully developed body, but 

 from all those ephemeral cells which arise and live for a while 

 during the ontogeny of the parent, performing certain special func- 

 tions and then making way for the definitive cells of the mature 

 organism. Thus in the germ-cells are stored actual physical repre- 

 sentatives of all the cells which have existed during the whole life 

 of the parent body. These innumerable gemmules remain inactive 



