100 



develop the green colour. Various Orthoptera, as the 

 common Blatta orientalis, emerge from the pupal skin white 

 or very pale, and only acquire the dark brown colour on the 

 hardening of the epidermal tissues. So I think it not un- 

 likely that the white patches in the Satyridce, the white spots 

 on the New Forest Argynnis paphia, and perhaps most ol 

 the pale and semialbine forms have arisen, not because the 

 pigment was absent — that is to say, could not have developed 

 because the elements of its formation were wanting — nor 

 because the once-formed pigment has been bleached in places, 

 but because, from some at present unknown cause, there has 

 been an arrest in the natural course of development of the 

 colour-granules. In the vertebrata the remarkable and often 

 localised effects of atrophy or disease of portions of the 

 nervous system are well known, and it is just conceivable 

 that some analogous cause may produce some of the effects 

 I have described in the invertebrata ; but such a theory 

 seems at present impossible of proof, and so is, at the best, 

 nothing more than a rather wild speculation, though it may 

 be that some proof exists unknown to me. 



There is no doubt, however, that animal pigments can 

 and often do change materially after their formation ; but 

 such changes seem only to occur after the death of the part 

 or the organism, or at any rate as a sign that the individual 

 is beyond the period of the greatest vigour and fullest life. 



Flowers, more especially yellow ones, as some of the 

 Cruciferce, will fade white, and this process may not unlikely 

 be. one of the breaking up of a complex yellow pigment into 

 some comparatively simple form that appears white, perhaps 

 the same, or nearly the same, as is seen in those Cruciferce 

 which normally have white flowers. Insects after death will 

 fade considerably on exposure to light for a long period, 

 and this, too, seems to be possibly due to a breaking up 

 of the pigments into simpler compounds. Certain cases of 

 paleness of colour or apparent albinism may be due to these 

 causes, and it will be necessary to be very careful to dis- 

 criminate between these and the true albinisms. For 

 instance, Helix nemoralis has a white variety; but a long 



