40 
The Action of Light upon the 
January, 
there is, no one has yet even attempted to show, and were 
such the case it would doubtless be traceable in a variety of 
phenomena not limited to the organic world. 
Another important point has been raised by Mr. Bates. 
He shows that whilst in many tropical butterflies the males 
are most splendidly coloured, the females — in numbers of 
cases at least — are sombre and insignificant in appearance, 
so much so that in former times they were often regarded 
as specifically distinct from their mates. If excess of light, 
therefore, be the producing cause of the splendour of the 
tropical Lepidoptera, why should not the effeCt appear alike 
in both sexes ? To this argument, however, the reply has 
been made that in these very species the females are exceed- 
ingly sedentary in their habits, remaining generally concealed 
in shady thickets, whilst the males flutter about in the sun- 
shine, and, being thus more exposed to light, experience 
modifications which — transmitted with constant accumula- 
tion from one generation to another — have produced the 
splendour now characteristic of their sex. To this question 
of the relative amount of exposure to light in different 
stages of existence we shall have to return. 
But the amount — or at least the intensity and clearness— 
of the sun does not necessarily vary with latitude alone. 
The air of some countries is more transparent, less obscured 
by fogs and clouds than that of others. More light evi- 
dently reaches the earth’s surface on open plains or on 
table-lands and in deserts than in dense forests and in narrow 
valleys Do we find any corresponding variation in the 
prevalent hues of the animal population of these respective 
localities ? Mr. Wallace points out that the most brilliantly- 
clad birds and inseCts are dwellers in the forests where the 
amount of light received is comparatively scanty. On the 
other hand, in the deserts, where — as we have already men- 
tioned — light must attain its terrestrial maximum, the 
prevalent colouration, if not dark, is certainly neither light 
nor brilliant. As the Rev. H. Tristram remarks, in such 
regions the smaller Mammalia, the birds, the snakes, and 
lizards are alike sand-coloured, their hues having evidently 
more reference to concealment than to the influence of an 
intense illumination. There is indeed, if we wish to come 
to details, a curious want of harmony in the effects which 
light is expected to produce. We know that it bleaches in 
certain cases and darkens in others ; but it is no easy task 
for us to predict when either of these opposite effects will 
be manifested. Still it is perfectly possible that light might 
have a bleaching power upon some living organisms, and a 
