less swarms. They have a host of enemies, and Buzzards, Owls, Ravens, and other predaceous birds thin their ranks by hundreds; while among four-footed foes, Polecats and Stoats follow the track of the advancing legions, and kill them where and when they can. The Polecat and Stoat are, moreover, able to follow the Hamster into the recesses of their burrows, where they probably destroy them by hundreds.[1]
Innumerable illustrations might be chosen from the life-histories of insects. Prof. Miall observes:—"Winter, of course, brings many hardships upon aquatic insects, as the great reduction in their number proves. The enormous number of eggs laid by so many of them is doubtless connected with the heavy risks to which they are exposed during half the year."[2] Of one of the May-flies (Polymitarcys virgo) Réaumur states:—"The short life of the winged female compels her to deposit her 700 or 800 eggs at once, without much discrimination of likely and unlikely places." Of the Pine Sawfly (Lophyrus pini), whose larvae are frequently found in such numbers in pine-woods, it has been said:—"When young, and also just before turning into pupæ, the grubs are very susceptible to sudden cold or heavy rain, which kill off thousands. In addition to these destructive agencies, nearly forty different kinds of parasites infest the grubs, while mice devour numbers of the pupæ."[3] Among the Threadworms (Nemathelminthes) parasitic Nematodes produce enormous numbers of eggs. Van Beneden states that 60,000,000 have been computed in a single Nematode, and this multiplication of ova is absolutely necessary, for the chance of the embryo reaching the right host, in which alone it can develop, is always a small one.[4]
This excessive fecundity in some animal life finds its parallel in plants. Thus it has been computed that a plant of Sisymbrium sophia yields 730,000, one of Nicotiana tabacum 360,000, one of Erigeron canadense 120,000, and one of Capsella bursa-pastoris 64,000 seeds yearly.[5] Probably in this case, and in a state of
