430 
Rev. H. B. Tristram on the 
animal contrives to maintain its existence. In the Desert, where 
neither trees, brushwood, nor even undulation of surface afford 
the slightest protection from its foes, a modification of colour, 
which shall be assimilated to that of the surrounding country, 
is absolutely necessary. Hence, without exception, the upper 
plumage of every bird, whether Lark, Chat, Sylvian, or Sand- 
grouse, and also the fur of all the small mammals, and the skin 
of all the Snakes and Lizards, is of one uniform isabelline or 
sand colour. It is very possible that some further purpose may 
be served by the prevailing colours, but this appears of itself a 
sufficient explanation. There are individual varieties in depth 
of hue among all creatures. In the struggle for life which 
we know to be going on among all species, a very slight 
change for the better, such as improved means of escaping from 
its natural enemies (which would be the effect of an alteration 
from a conspicuous colour to one resembling the hue of the 
surrounding objects), would give the variety that possessed it a 
decided advantage over the typical or other forms of the species. 
Now in all creatures, from Man downwards, we find a tendency 
to transmit individual varieties or peculiarities to the descend¬ 
ants. A peculiarity either of colour or form soon becomes here¬ 
ditary when there are no counteracting causes, either from 
change of climate or admixture of other blood. Suppose this 
transmitted peculiarity to continue for some generations, espe¬ 
cially when manifest advantages arise from its possession, and 
the variety becomes not only a race, with its variations still more 
strongly imprinted upon it, but it becomes the typical form of 
that country. If it be objected that we see many varieties which 
do not become hereditary, we may reply, that these varieties 
having experienced changes not advantageous to their means of 
existence, may from that very cause become extinct. Still there 
are many which continue, as the Pied Raven of the Faroe 
Islands, and the Tailless Manx Cat. 
To apply the theory to the case of the Sahara. If the Alge¬ 
rian Desert were colonized by a few pairs of Crested Larks,— 
putting aside the ascertained fact of the tendency of an arid, hot 
climate to bleach all dark colours,—we know that the proba¬ 
bility is, that one or two pairs would be likely to be of a darker 
