206 DARWINISM TO-DAY. 



less rapid of others, but, on the contrary, for each species 

 there is a definite law of growth which we may only explain 

 as the expression of a force of heredity, not capable yet of 

 analysis. Matters certainly do not go on in an embryo as 

 in an agar culture containing several kinds of bacteria of 

 which only that one with the greatest life force remains. 

 Were the development of the embryo determined by the 

 food-zeal of the cleavage cells, it would happen that in a 

 short time a few cells specially capable of assimilation would 

 get the upper hand, and as a consequence only a few quali- 

 ties be left to the embryo; a real differentiation into thou- 

 sands of different cell-sorts would not be possible. All the 

 facts of symmetry and auto-regulation in embryonic develop- 

 ment speak against any considerable influence of a battle of 

 the parts during development. 



Fourth, Plate declares that in the acquirement of new 



characters no selective intra-struggle takes place, or at least 



in only most insignificant manner, but that the 



the parts in the new structures arise either through the direct 



acquirement of influence of new stimuli or by natural selection 



new characters, ,. . . 



of new germinal variations of unknown origin. 

 In the first place it is simply the matter of position, not at 

 all that of quality, that decides whether the certain cells 

 shall be changed or not. Think, for example, of a vessel 

 in whose walls the connective tissue fibres cross and recross 

 in all directions wholly without order, and conceive that a 

 constant or repeated stress in both longitudinal and trans- 

 verse directions is exerted on this vessel. It would result 

 that all those fibres lying in the absolute or approximate 

 directions of these stresses would be most stretched and 

 would in consequence of their trophic irritability most rapidly 

 enlarge and increase with special rapidity. Now by the 

 repetition and inheritance of this result of use it would finally 

 come about in the course of generations that all the fibres 

 situated in other directions to the stresses would die out,. 



