APPENDIX G. 



THE INHERITANCE OF FUNCTIONALLY-CAUSED 

 MODIFICATIONS. 



IN Part II, Chapter X A , I have confessed that the process by 

 which a structure changed by use or disuse affects the sperm- 

 cells or germ-cells whence arise descendants, is unimaginable : 

 without, however, inferring that therefore such a process does 

 not exist. With others it seems different. Some three years 

 ago the following expression of opinion came to me from a 

 zoological expert : 



" Many zoologists most of us here at Cambridge are intensely opposed 

 to the doctrine of the inheritability of acquired variations. Even assuming 

 that the developmental power of a germ is determined by its molecular 

 structure (and I for one would question this Driesch and his school when 

 they find that they can squeeze a developing egg into all sorts of shapes 

 without altering the final result, that one blastomere in an egg which has 

 divided into 8 is still able to reproduce a whole embryo question it also), 

 we still fail to conceive any means by which, for instance, a change in the 

 development of a muscle or nerve can effect a corresponding change in that 

 part of the germ which is destined to produce a corresponding part in the 

 descendant." 



Here it will be observed that belief in the inheritance of 

 structural effects wrought by use and disuse, is rejected because 

 of inability " to conceive any means " by which the modifications 

 produced in an organ can effect a correlated modification in the 

 germ of a descendant : failure to conceive is the test. The im- 

 plication is that some alternative hypothesis is accepted because 

 the correlating of a variation in an organ with a corresponding 

 germ-variation is effected by a means which is conceivable. This 

 is the hypothesis of Weismann. Concerning its conceivability 1 

 have, in the chapter just named, already written as follows : 



" If we follow Prof. Weismann we are led into an astounding supposition. 

 He admits that every variable part must have a special determinant, and 

 that this results in the assumption of over two hundred thousand for the 

 four wings of a butterfly. Let us ask what must happen in the case of a 

 peacock's feather. On looking at the eye near its end, we see that the 

 minute processes on the edge of each lateral thread must have been in some 

 way exactly adjusted, in colour and position, so as to fall into line with the 

 processes on adjacent threads : otherwise the symmetrical arrangement of 

 coloured rings would be impossible. Each of these processes, then, being an 

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