88 PROBLEMS OF GENETICS 



stimuli" which naturalists who incline to regard adaptation as 

 a direct consequence of an environmental influence might most 

 readily invoke as an illustration of their views. And yet all 

 evidence is definitely unfavourable to the suggestion of an 

 inheritance of the acquired power of resistance. Such change as 

 can be perceived in the virulence of the attacks on successive 

 generations may be most easily regarded as due to the exter- 

 mination of the more susceptible strains, and perhaps in some 

 measure to variation in the invading organisms themselves, an 

 "acquired character" of quite different import. 



The specific "anti-body" may have been produced in re- 

 sponse to the stimulus of disease, but the power to produce it 

 without this special stimulus is not included in the germ-cells 

 any more than a pigment. All that they bear is the power to 

 produce the anti-bodies when the stimulus is applied. 



If we could conceive of an organism like one of those to 

 which disease may be due becoming actually incorporated with 

 the system of its host, so as to form a constituent of its germ-cells 

 and to take part in the symmetry of their divisions, we should 

 have something analogous to the case of a species which acquires 

 a new factor and emits a dominant variety. When we see the 

 phenomenon in this light we realise the obscurity of the problem. 

 The appearance of recessive varieties is comparatively easy to 

 understand. All that is implied is the omission of a constituent. 

 How precisely the omission is effected we cannot suggest, but 

 it is not very difficult to suppose that by some mechanical fault 

 of cell-division a power may be lost. Such variation by unpack- 

 ing, or analysis of a previously existing complex, though unac- 

 countable, is not inconceivable. But whence come the new 

 dominants? Whether we imagine that they are created by some 

 rearrangement or other change internal to the organism, or 

 whether we try to conceive them as due to the assumption of 

 something from without we are confronted by equally hopeless 

 difficulty. 



The mystery of the origin of a dominant increases when it 

 is realised that there is scarcely any recent and authentic account 

 of such an event occurring under critical observation, which can 



