OUR NATURAL INHERITANCE 73 



descendants tend to be eventually all grey like the 

 wild rabbit. " Darwin regarded this as a reversion, 

 and it may still be described as reversionary ; but it is 

 not due to the reassertion of long latent grey colouring. 

 The return to grey is due, as the Mendelian experi- 

 ments show, to the recombination of at least eight 

 colour-ingredients (' factors ' or ' genes ') that go to 

 the make-up of the wild greyness. Man has sifted 

 out all the various colours from the complex colora- 

 tion of the wild stock, and when the long-separated 

 items are brought together again by unrestricted inter- 

 breeding there is, naturally enough, a reconstruction 

 of the original grey colouring " (see the author's Dar- 

 winism and Human Life, p. 147). 



§ 6. Inheritance and Disease 

 There is much still to be learned in regard to the in- 

 heritance of disease, defects, abnormalities, and the like, 

 but the following propositions are probably justifiable. 

 (a) The reappearance of a diseased condition, like 

 rheumatism, in a lineage, does not prove that it has 

 been transmitted, or that it is transmissible. In the 

 caves of Dalmatia, Carinthia, and Carniola there is a 

 wan white blind newt called Proteus. It shows no 

 pigment in its skin, and the only spot of colour other 

 than white is where the red blood shines through the 

 delicate gills. It would seem to be a very safe con- 

 clusion to say that Proteus is hereditarily pigmentless. 

 And yet if it be exposed to light, it becomes rapidly 

 dark. It is as sensitive as a photographic plate. What 



