Chap. XXV. CORRELATED VARIABIIJTV. 323 



*' eye, as I have several times observed, be not blue, the cat 

 " hears. On the other hand, I have never seen a white cat 

 " with eyes of the common colour that was deaf." In France 

 Dr. Sicliel -^ has observed during twenty years similar facts; 

 he adds the remarkable case of the iris beginning, at the end 

 of four months, to grow dark-coloured, and then the cat first 

 began to hear. 



This case of correlation in cats has struck many persons 

 as marvellous. There is nothing unusual in the relation be- 

 tween blue eyes and white fur ; and we have already seen 

 that the organs of sight and hearing are often simultaneously 

 affected. In the present instance the cause probably lies in 

 a slight arrest of development in the nervous system in con- 

 nection with the sense-organs. Kittens during the first nine 

 days, whilst their eyes are closed, appear to be completely 

 deaf; I have made a great clanging noise with a poker and 

 shovel close to their heads, both when they were asleep and 

 awake, without producing an}^ effect. The trial must not 

 be made by shouting close to their ears, for they are, even 

 when asleep, extremely sensitive to a breath of air. Now, as 

 long as the eyes continue closed, the iris is no doubt blue, 

 for in all the kittens which I have seen this colour remains 

 for some time after the eyelids open. Hence, if we suppose 

 the development of the organs of sight and hearing to be 

 arrested at the stage of the closed eyelids, the eyes would 

 remain permanently blue and the ears w^ould be incapable of 

 perceiving sound ; and we should thus understand this curious 

 case. As, however, the colour of the fur is determined long 

 before birth, and as the blueness of the eyes and the whiteness 

 of the fur are obviously connected, we must believe that some 

 primary cause acts at a much earlier period. 



The instances of correlated variability hitherto given have 

 been cLiefly drawn from the animal kingdom, and we will 

 now turn to plants. Leaves, sepals, petals, stamens, and 

 pistils are all homologous. In double flowers we see that 

 the stamens and pistils vary in the same manner, and assume 

 the form and colour of the petals. In the double columbine 

 [Aquilegia vulgaris), the successive whorls of stamens are con- 



2* 'Annales des Sc. N^at.' Zoolog., 3rd series, 1847, torn. viii. p. 239, 



