52 PERIODICITY IN ORGANIC LIFE.
of definite duration however, when the individuals of a species are
very abundant or the reverse, scarce and apparently extinct, or nearly
so. The scarcity or abundance in any year of insect life would
appear to be quite independent of seasons, or of food-supply, or of
any other surroundings, as far as our present knowledge extends.
True, there are occasionally years of great heat, and therefore of
presumably favourable effect on animal life, but it is not to these
years and their effects that I wish to draw the attention of naturalists ;
the explanation of the abundance of life in such periods being so
obvious to all. It is rather to the fact that there are periods in the
existence of every species, when from no apparent cause the indivi-
duals gradually increase year by year until we have them in great
abundance.
After this period of what may be termed prosperity of a species,
which may continue for some years, a decline begins, when the
numbers gradually go down until it would appear that the species
becomes nearly extinct. This period of scarcity may also be of
some long continuance, or it may possibly lead to the extinction of
the species in reality. The extinction of any species of insect by
artificial causes very rarely happens, but is much more common
amongst mammals and birds, especially with those creatures having
a limited habitat. The case most frequently quoted is that of the
Dodo, but there are many others equally decisive, and as clearly due
to man's interference with Nature.; such as the Rhytina in Behring's
Island, the Moa in New Zealand, and the last, and the most dis-
graceful case of wanton destruction, that of the Bison in North
America, and many others.
One insect is said to have been destroyed during the last few
years by the hand of man. I refer to the Great Copper Butterfly
(Polyommatus dispar). It is said that the drainage of Whittlesea
Mere, by destroying the plant on which the caterpillars fed, caused
the extermination of the butterfly. This may have been so ; but I
very much doubt the assertion. The habitat certainly was not con-
fined to Whittlesea Mere, and even if it were, the food-plant (the
(Great Water Dock, Rumex hydrolapathum) abounded everywhere in
the district where sufficient water was present. On this it might
have remained, perhaps in greatly diminished numbers, and it may
possibly now be still existing. Although the grand butterfly has not
for some years gladdened the eyes of the collector, I should never
be surprised to hear that it had been again discovered in some