It may be well to pause a moment here and ask why it is that animals are not completely exterminated which are thus perpetually being persecuted and preyed upon by stronger foes. Why, for instance, are any frogs left in a green locality, when they are so eagerly sought for by their enemies, the snakes? What are the factors that permit the average or normal number of them to exist in the given locality? The most important of them have already been referred to separately, but it will be useful to recapitulate, briefly, a few of them now. One of the most important of them is the tendency of the frogs to increase in geometrical ratio. In the breeding season great numbers of young are brought into the world, thus reinforcing the depleted ranks of the adults. Another factor is the instinctive effort of each frog to avoid its enemies, and their protective coloration greatly facilitates their concealment and escape. Another factor is the circumstance that the snakes also have enemies. They must be wary and careful in hunting prey lest they be unduly exterminated by their foes, the snake-eating animals. Then another factor is the alternating day and night, by which the warfare, offensive and defensive, must be periodically checked. Then again, the hibernating season, winter, checks or rather suspends the life-destroying crusades. It may thus be recognized that many complex factors occur to explain the circumstance that a given locality generally contains the normal number of individuals that constitute the species of frogs. The complex factors at work in these relations and inter-relations of animals and plants are so nicely balanced that with a comparatively stable environment the number of species of the different classes of animals may remain more or less constant. But suppose one of these factors is profoundly diminished, such, for instance, as a great diminution in the number of the snake-eating birds. Then the snakes, not being much persecuted by their enemies, will be in a most favorable environment; they will increase greatly in numbers; they will be eagerly on the look-out for their food, the frogs; the frogs, being thus unusually persecuted and hard-pressed, will diminish greatly in numbers. Now, this will be a golden opportunity for the insects; their environment becomes much more favorable. The snakes do not bother the insects, but destroy their enemies, the frogs. Therefore, the insects increase greatly in numbers and profoundly affect the vegetation upon which they live and thrive. The injury to the vegetation may thus seriously affect an altogether different class of animals—the grass-eating or foliage-eating animals, like the ruminants.