EXTENSION OF EVOLUTIONARY THEORIES 333 



selection, Weismann, has attempted to apply it to the 

 germ-cells.^ He thinks that here also there is a selection 

 at work — "germinal selection." The result of this is to 

 preserve definite directions of variation in the germs. 



We have already seen several times that there are 

 in the germ -cells, which an animal has in its sexual 

 glands, tiny particles that represent the rudiments of 

 the future organs, in the sense that, if an animal is 

 developed from the germ-cell, they determine the several 

 parts of the body and stimulate their construction. 

 Hence every part of an animal's frame is present 

 rudimentarily in the germ-cell. 



These basic particles, which Weismann calls deter- 

 minants, because they afterwards determine an organ 



^ Weismann was influenced by the " theory of histological selection " 

 of Wilhelm Roux. Roux attributed the purposive structure in the 

 histological, microscopic texture of animals to " a struggle among their 

 elements." We know that sustained exercise or stimulation in the life 

 of the individual strengthens. The cells are enabled to grow better 

 under stimuli. Hence when we find a graceful framework in the bones 

 of the leg, the arches of which always run in the direction of the 

 greatest pressure and strain, as in the construction of an edifice, it must 

 have been brought about by the fact that the cells which lay in the 

 direction of the pressure and strain were most stimulated, and so 

 formed the strongest bony matter. On the other hand, the intermediate 

 cells were less stimulated, would grow and act less, and the nourish- 

 ment they needed would be absorbed by the better situated cells, so 

 that they would eventually perish. It is thus that the curves in the 

 osseous structure run only in the direction of the greatest strain. The 

 cells that lie there have an advantage in the struggle of parts, as they 

 grow under the constant stimulation, and build up bony substance 

 at the expense of the adjacent cells, which gradually die off. 



Histological selection gives us a luminous explanation of microscopic 

 structure. But in our opinion it only acts during individual develop- 

 ment. What it accomplishes is not inherited, but has to be created 

 afresh in each organism. 



