ROSS'S POSTULATE 147 



of the nervous system, or, again, in susceptibility to certain 

 specific diseases which are peculiarly apt to affect man, relative 

 immunity toward which on the part of the healthy individual 

 must be regarded as a comparatively recent acquirement. As 

 my old teacher, James Ross, of Manchester, was the first, I 

 believe, to point out, when there is progressive atrophy of the 

 cells in the cortex of the brain the first motor cells to show signs 

 of that atrophy are those governing the muscles which differentiate 

 man from other animals, namely, the opponentes muscles of 

 the hand. 



Hence it has to be laid down that the attachment of these 

 side-chains, which are recently acquired, is relatively unstable, 

 that such recent side-chains are most easily detached, so that 

 the idioplasm, and the organism developed around that idioplasm, 

 are prone to return to their former condition or lose characters 

 gained by recent progenitors. The more side-chains become 

 attached, the more complicated the structure, the more unstable 

 the equilibrium ; hence the greater the liability to revert. As 

 Professor Walker points out to me, parallel conditions are to 

 be recognized in organic chemistry. The old-established view 

 of the existence of " radicles " to a large extent admits this 

 principle ; certain central atomic groups in a compound are 

 seen to be more fixed and not to undergo change when the 

 attached groups are removed and replaced. Still more closely 

 allied is what has been observed in connexion, for example, 

 with aniline. The composition of this is C 6 H 5 NH 2 . Here the 

 H atoms of the NH 2 group can be replaced by other groups, 

 by methyl (CH 3 ) and ethyl (C 2 H 5 ) groups, etc., and it is found 

 that under certain conditions, in carrying out a series of these 

 replacements, the last group attached is the first to be split off 

 and replaced. 



We see this principle in action in the lowest forms of life. 

 Contrary to what Weismann has laid down regarding partheno- 

 genesis and asexual reproduction, it is comparatively easy to 

 impress new characters upon the bacteria. By subjecting a 

 growth of pigment-producing bacteria to the action of a tem- 

 perature just below that which will cause their death, we can 

 bring about a loss of pigment production, so that the rapidly 

 succeeding generations are perfectly colourless, but gradually in 

 the course of time the cultures made from the original (heated) 



