How clearly we can see in all this that the tendency to multiply rapidly, so advantageous in normal seasons, becomes almost fatal to a species in seasons of exceptional abundance. Cover and food without limit enabled the mice to increase at such an amazing rate that the lesser checks interposed by predatory species were for a while inappreciable. But as the mice increased, so did their enemies. Insectivorous and other species acquired the habits of owls and weasels, preying exclusively on them; while to this innumerable army of residents was shortly added multitudes of wandering birds coming from distant regions. No sooner had the herbage perished, depriving the little victims of cover and food, than the effects of the war became apparent. In autumn the earth so teemed with them that one could scarcely walk anywhere without treading on mice; while out of every hollow weed-stalk lying on the ground dozens could be shaken; but so rapidly had they devoured, by the trained army of persecutors, that in spring it was hard to find a survivor, even in the barns and houses. The fact that species tend to increase in a geometrical ratio makes these great and sudden changes frequent in many regions of the earth; but it is not often they present themselves so vividly as in the foregoing instance, for here, scene after scene in one of Nature's silent passionless tragedies opens before us, countless myriads of highly organized beings rising into existence only to perish almost immediately, scarcely a hard-pressed remnant remaining after the great reaction to continue the species. CHAPTER IV. SOME CURIOUS ANIMAL WEAPONS. Strictly speaking, the only weapons of vertebrates are teeth, claws, horns, and spurs. Horns belong only to the ruminants, and the spur is a rare weapon. There are also many animals in which teeth and claws are not suited to inflict injury, or in which the proper instincts and courage to use and develop them are wanted; and these would seem, to be in a very defenceless condition. Defenceless they are in one sense, but as a fact they are no worse off than the well-armed species, having either a protective colouring or a greater swiftness or cunning to assist them in escaping from their enemies. And there are also many of these practically toothless and clawless species which have yet been provided with other organs and means of offence and defence out of Nature's curious armoury, and concerning a few of these species I propose to speak in this place.