Powerful and irresistible is the tendency to generate; in a great many cases the individual sacrifices himself consciously, or is unwittingly sacrificed by the laws of nature, provided that before death he transmit life to others. "Let the individual perish, if this preserve the species!" Such is the eternal cry of nature, which men and infusoria, oaks and mushrooms alike must obey. If the individual is protected and possesses preservative instincts and defensive organs, the species has a hundred bulwarks, a thousand manners of safeguard, more means of protection than are needed. In fact, living beings generate so profusely that one species alone would pervade the earth if the various circles of expansion, falling in with each other, did not struggle among themselves, like the circles caused on the smooth surface of a lake by a handful of sand thrown upon it by a child. Apart from the manner in which life is transmitted, there is an amount of life which passes away, there is _a certain amount_ of fecundity, and this may seem, at first glance, most whimsical, while it is governed by the laws of preservation. _To be born and to die--fecundity and mortality_--are so closely connected with each other that we can consider them as different aspects of the same phenomenon, as the action and reaction of life. When reproduction increases beyond measure, the dangers for the individuals generated increase at the same time, and destruction mows down the excessive number of those which are born. Now it is food that is no longer proportionate to the new-born; then parasites and enemies of the over-expanded species, which, increasing in turn, reëstablish the equilibrium. The destructive forces and the protective balance mutually, as happens with many other forces, simpler and better known. The Malthusian problem, however, is much more intricate. If all species were equally prolific and had a life of equal length, the problem would, in fact, be reduced to a question of space and food; but, on the contrary, the duration of life and the various degrees of fecundity serve in turn to reëstablish the equilibrium by other ways. If the reproduction of mice were as slow as that of man, they would all be destroyed before another generation could be born; and even if they could live fifteen or sixteen years, not one of them, perhaps, would ever attain that age, surviving all dangers. And on the other hand, should oxen multiply in the same proportion as infusoria, the entire species would die of hunger in a week.