Heredity and the Origin of Variations. 161 



tory movements " or " polarities " does not enable us to 

 comprehend with any definiteness how this particular modi- 

 fication of these particular nerve-cells is so conveyed to the 

 germ that it shall produce an organism with analogous 

 nerve-cells modified in this particular way. 



(c) The hypothesis that the germ-plasm may be con- 

 verted into body-plasm, which, on its return again to the 

 condition of germ-plasm, may retain some of the modifi- 

 cations it received as body-plasm, seems to be negatived, 

 so far as most animals are concerned, by the facts of 

 embryology and development. The distinction of germ- 

 plasm and body-plasm I hold to be mythical. And there 

 is no evidence that cells specially differentiated along 

 certain lines can become undifferentiated again, and then 

 contribute to the formation of ova or sperms. From the 

 view-point of cell-differentiation, which seems to me the 

 most tenable position, there does not seem any evidence 

 for, or any probability of, the occurrence of any roundabout 

 mode of development of the germinal cells which could 

 enable them to pick up acquired characters en route. 



(d) We come now to the contention that the organism, 

 being one and continuous, if any member suffers, the germ 

 suffers with it. The organs of the body are not isolated or 

 insulated ; the blood is a common medium ; the nerves 

 ramify everywhere ; the various parts are mutually de- 

 pendent : may we not, therefore, legitimately suppose that 

 long-continued modification of structure or faculty would 

 soak through the organism so completely as eventually to 

 modify the germ ? The possibility may fairly be admitted. 

 But how is the influence of the body brought to bear on 

 the germ? The common medium of the blood, proto- 

 plasmic continuity, the influence of the products of chemical 

 or organic change, these are well enough as vague sug- 

 gestions. But how do they produce their effects ? Once 

 more, how is this increased power in that biceps muscle 

 of the oarsman able to impress itself upon the sperms or 

 the ova ? No definite answer can be given. 



We are obliged to confess, then, that no definite and 



M 



