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tratcd by the fact that, while certain species have a tendency to vary in the above 
and other manners, few species arc liable to the same extent of variation, and 
many appai'eutly to none at all. 
Of course it must be understood that originally these hereditary peculiarities 
hiivo been acquired through somo accident, or by the force of surrounding con- 
ditions, and have in the course of generations become perpetuated in what ia 
termed a variable species, or in a race of individuals presenting appreciable differ- 
ences from the typical form. 
Second. — We have what may be called topographical influences ; influences 
acting in greater or less degree on the fauna of a district through the nature of 
the locality ; and among these the soil especially would appear to exert a potent 
influence, since we find certain species varying according as they may have repro- 
duced, generation after generation, on a chalky, peaty, gravelly, or other soil. This 
cause may act indirectly through the vegetation of the respective districts which 
doubtless undergoes some modification. 
Next, we have food, and whatever influence tliis may exert must naturally bo 
through the larva ; but of the fact that it does operate in various ways upon 
the future imago we have ample proof. The colour of the perfect Tortrix viridana 
is a familiar example of the power of food to produce variation in the imago, and 
there are many other instances of so-called phytophagic species, races, or 
varieties, chiefly among the Micro-Lepidoptera, which might be adduced. 
It must not, however, be understood that, as a rule, changes of this kind are 
wrought in one or even in ten generations. We see, too, alteration in the colour of 
the larva brought about by the agency of food ; thus in that ofEwpithecia ahsyn- 
thiata and of other species, there is a tendency to assume the tint of whatever 
flower they may be feeding upon ; and again, the colour of the silk of which the 
cocoon of the Hcdias prasinana is formed, depends on whether the larva had pre- 
viously fed on oak, hazel, &c. We see, too, that species single brooded on one plant 
have a tendency to become double brooded on another, as in the case of Orgyia 
gonostigma reared respectively on oak and willow or sallow. And, finally, with 
respect to that unsatisfactory pet of the variety breeder — Arctia caja — many dark 
specimens have been attributed to the agency of coltsfoot, lettuce, and other 
pabula; but the late Peter Bouchard used to say of one of his best varieties of 
" the tiger," that he could account for it in no other way than that the beast must 
have lunched ofi" some bread and cheese which had accidentally fallen into its den. 
Then come causes of variation produced by the action of light (upon the 
larval and pupal stages especially), through which it is averred the future imago 
is rendered darker or paler, according as this influence may have been intense and 
prolonged, or nearly wanting, and of short duration. It is certainly worth the 
wMle of the variety-breeder to act upon and tost the value of light as an agent in 
causing vai'iation. 
And, lastly, we come to those influences which operate chiefly upon the pupa ; 
of these, perhaps, the atmospheric hold the foremost place, and under these may 
bo classed what may bo denominated the " thermic," causing retardation or ac- 
celeration of the completion of this stage ; and this, like other causes of variation, 
appears to affect the individuals of certain species more than of others ; for while. 
