PRESENT ASPECTS OF EVOLUTION 303 



veloping somatic cells, moulding them iuto appropriate 

 organs, and upon germ-cells in process of formation, 

 moulding them so that they may continue its sway. 

 The muscle, modified by use or disuse, is a better ex- 

 pression of the individuality of its possessor, and the 

 same individuality moulds similarly and simultaneously 

 the germ-cells. Both are different expressions or man- 

 ifestations of the same individuality. Only slowly 

 does the individuality mould the muscles and nerves 

 of the adult body to its use. Still more slow may be 

 the moulding of the still more refractory germ-plasm, if 

 such there be. But the moulding process goes on 

 parallel in the two cases. 



But Weismann's argument rests not merely upon 

 any difficulty or impossibility of the transmissibility 

 of acquired characteristics. His argument is rather 

 that all facts can be better explained by his theory 

 without postulating or accepting such transmission, 

 cases of which have never been absolutely proven. 

 But the question is not whether his theory offers a 

 possible explanation of the facts, but whether it is the 

 most probable explanation of all the facts. No one 

 would deny, I think, that the continuity of the germ- 

 plasm offers the best and most natural explanation of 

 heredity ; and that variations could be produced by 

 the influence on the germ-plasm of external conditions 

 seems entirely probable. 



But when we consider the aggregation of these vari- 

 ations in a process of evolution, his theory seems un- 

 satisfactory. We have already seen that what we 

 commonly call a variation involves not one change, but 

 a series of changes, each term of which is necessary. 

 Muscle, nerve, and ganglion must all vary simultane- 



