47 
species. On the tree-trunks on which it rests, its colour and markings 
are its salvation. Yet its colour would be fatal on the black fences 
and tree-trunks to be found in all manufacturing districts. In these 
districts, natural selection has eliminated the pale conspicuous forms, 
and a melanic form known as the var. doubleday aria has been evolved 
in its place. The process of evolution has been exceedingly simple, 
just the weeding out of the most conspicuously pale specimens, 
and the retention of the darker and less conspicuous forms. The same 
process of selection has taken place in the formation of the melanic 
aberrations of Tephrosia crvpuscularia, T. bistortata, Diurnaea fayella, 
and numbers of other species. Utility is the mainspring of the forma¬ 
tion of all these melanic forms. The physiological factors of variation 
necessary for the production of this result were (and are) present in all 
these species. They possess, in their typical forms, black and white 
scales in varying numbers; utility has seized on the useful character, 
and has moulded the material at its disposal into its own channels for 
the advantage of the species. 
Again, let us examine a species like Gnophos obscurata. This 
species, all lepidopterists are aware, rests upon the ground, and is 
entirely dependent for its protection on the resemblance which its 
colour bears to that of the rocks upon which it rests ; and this 
resemblance is perfect—black on peat and dark slate, grey on lime¬ 
stone, white on chalk, with such a nice gradation in tint, corresponding 
with that of the different rocks upon which the species is found, that 
one can almost tell exactly where individual specimens have been 
captured. Here, again, the part that utility has played in the deter¬ 
mination of the various local races of this species is obvious. 
I do not here wish to enter into the physiological processes in¬ 
volved in the development of these local forms. I only want to point 
out that they have been developed, because the points which distinguish 
them from each other and the type, are severally useful to the species, 
under those conditions of environment by which each individual form 
finds itself surrounded. 
The modification of many of the species here incidently referred 
to —Amphidasys betularia, etc.—in the direction of melanism is largely 
connected with certain habits that these species possess in common. 
The phenomenon, too, is undoubtedly of comparatively recent 
occurrence, and has largely increased within the last fifty years. 
That the phenomenon, as exhibited by these species, has been brought 
about by changes in the environment, must be obvious to all who will 
only look for them. The modification of other species - — Gnoplios 
obscurata, Dasydia obfuscata, Ayrotis lucernca , etc.—with a slightly 
different habit, sometimes, in a somewhat similar direction, is of 
much greater age, and dates back probably as long as the species have 
rested on differently coloured rocks in different localities. Still, the 
hand of utility is just as evident here, as in the previous cases. What 
is true of the formation of these local races, must be true in a 
measure of species themselves. 
ON THE ORIGIN OF SPECIES. 
A great change in the environment of a species, locally, produces 
in that district a local race. It matters not whether the change be 
one of climate, food-supply, introduction of new enemies, or geological 
change, so long as it is sufficiently marked to affect the species in- 
