26 
pupae go over several years until a favourable season arises, that they 
are generally kept in check by parasites and only occasionally get the 
upper hand, or that they are occasionally augmented by immigration, 
are among the commonplaces of entomology. None of these factors 
which I have mentioned are likely to be altogether inconsiderable, and 
some, at least, have been in a measure raised out of the realm of mere 
theory into that of demonstrated fact; but the difficulty, to the thought¬ 
ful mind, is that they do not go back to first causes, as, for instance, 
those which regulate the migrations, or those which regulate the 
balance between parasites and their hosts. I suppose we must not be 
surprised at this difficulty in getting to the very heart of the problems, 
however dissatisfied we may be at our inability to lay out for ourselves 
a clear and comprehensive theoretical scheme ; no one can have 
thought seriously for five minutes over the operations of natural 
selection, and the inter-relations between the various organisms and 
forces of organic (and even of inorganic) nature, without having 
realised how excessively complicated they are, and how multifarious 
the factors may well be which make for the survival or the dominance 
of this or that individual form. 
Perhaps all that we can venture to assert at all dogmatically is that 
a particular species will be abundant just when and where there is 
room for it, in the so-called “ economy of nature.” But this assertion 
is little better than a begging of the question, as it does not attempt 
to show what special concatenation of circumstances will open up the 
path or clear the way for a species, so to speak, at this or that period. 
It has been generally remarked, and is plausible enough, that an in¬ 
crease of the right foodplant, or the introduction of the foodplant in 
a new locality, has a marked influence on the increase or spread of the 
species thus provided with the needful pabulum. Even such an old- 
fashioned entomologist as Newman, in his British Moths, emphasizes 
the correlation between the cultivation of turnips and the increase of 
that pest of the farmer (but pet of the student of variation), A<irotis 
seaetum. Probably we can all add examples from our own experi¬ 
ence. I myself have noticed the fluctuations in the commonness or 
scarcity of the pretty little Geometrid Pernoniaalcheinillata in accord¬ 
ance with those of its somewhat erratic foodplant, Galeojisis tetrahit 
(common hemp-nettle). Our botany books tell us that this is a 
common weed, and from the little acquaintance which I have of 
Scotland I should say that that is so indeed in the North ; but at 
Sandown it is, in my experience, as I have just described it, a “ some¬ 
what erratic ” plant. Some seasons I have hunted round the hedges 
on the farm fields and hardly found a scrap, while in other years it 
is in absolute profusion in its favourite fields. Now in those years 
when it is scarce, one might be inclined to expect that the little which 
was available would be crowded with larva) of /’. alchemillata — just as 
we do indeed often find, in the case of an isolated buckthorn, that 
there is an excellent chance of reaping a big harvest of ('alias rhanmi 
lame; but in the case under notice, 1 have by no means found this 
so, but on the contrary have even fancied that the lame were /irajiar- 
tionalhj Scarce in the years of dearth of the Ualenpsis, while they 
certainly abound in its years of plenty, so that one cannot help think¬ 
ing that there is a vital connection, though of course it might amount 
to nothing more than that the seasonal circumstances which favoured 
