28 
the normal one, we have one individual protected to the average 
degree, and nine not adequately protected against the particular 
dangers that are in abeyance. Nay, it is very possible that these 
special dangers left not ten, but twelve or fifteen individuals, and that 
the other destructive agencies reduced the twelve or fifteen to ten, 
including in the destruction the one specimen that was protected to 
the ordinary survival standard against the absent dangers; in that 
case all ten individuals would be below normal survival standard in 
this direction. It is obvious that supposing one in ten of the more 
numerous ten-fold population were really of the highest selected type, 
it would nevertheless be nine to one against its pairing with a similar 
individual, so that instead of one in 100 being of this type in each 
generation as normally, now there will be only one in 10,000 in the 
following year, and in the third year the proportion would be infini¬ 
tesimal. The next generation would inherit this reduced protection, 
and the 100 that then survived would be again much less protected 
than their parents. They would all be individuals that would usually 
have perished, and to a greater degree than nine of their parents (or 
of the whole ten). 
Whatever may be the precise rate at which this process would go 
on in any individual case, whether it took one, two, or half a dozen 
years, and 'whether the increase in the abundant year was 10 or 100 
fold, a process of the nature I have just suggested must have recurred, 
and the whole swarm probably does not present one individual of the 
highly selected character of an ordinary year. 
Should it so happen that one of the selecting forces temporarily in 
abeyance selected on a matter of colour or markings of the imago, 
say the absence of some bird, or some similar condition, then these 
unselected moths would show more variation in these colours or 
markings than occurs in an ordinary year. In the following year, 
the individuals would be still less protected ; now let the destructive 
agencies, against which the individuals that exist are protected to a 
de gree much below the normal necessity, resume their full vigour, the 
whole race will be practically wiped out, and instead of the normal 1,000 
survivors, there will be only a few, say half a dozen, that have, as 
it were, escaped by accident. It will take some time for this degenerate 
race to be selected up to normal condition again, in fact, one would 
expect that if the destructive agencies, against which protection has 
been so much diminished, resumed their full power suddenly, and 
continued at concert pitch, extinction must be the result, and the district 
affected would therefore only be repopulated by immigrants. If the 
enemy were a bird or ichneumon, it would hardly resume full vigour 
suddenly, but a climatic variation might have the full effect of a 
sudden resumption of destructive energy—the result would then be a 
complete wiping out of the whole race. 
If this speculation, or rather calculation, of the effect of selection 
being temporarily slackened be correct, it must of course be in con¬ 
tinuous action, and assuming such fluctuations of selective forces, it 
would seem to follow that whenever a species became in this way more 
abundant, it would eventually be rendered proportionally less abundant 
in the next or some early year, unless the variation of the selective 
forces were very gradual, extending over some years, and were not 
great in amount. 
xix. 
