This is another fact of Nature which requires to be kept in mind in all discussions of the action of natural selection, yet it is often altogether ignored by critics of the theory. As an illustrative fact, a not uncommon European weed of the Cruciferæ family has been found to produce about 700,000 seeds on a single plant, whence it can be calculated that if every seed had room to grow for three successive years their produce would cover a space of about 2,000 times as large as the whole land surface of the globe. Some of the minute aquatic forms of life which increase by division in a few hours would, if they all had the means of living, in the same period occupy a space equal to that of the entire solar system. Even the largest and slowest breeding of all known mammals, i.e. the elephant, would, if allowed space to live and breed freely for 750 years, result in no less than nineteen million animals. By far the larger part of the criticisms of Darwinism by popular writers are due to their continually forgetting these two great natural facts: enormous _variability_ about a _mean value_ of every part and organ; and such ever-present powers of multiplication that, even in the case of vertebrate animals, of those born every year only a small proportion—one-tenth to one-hundredth or thereabouts—live over the second year. If they all lived their numbers would go on continually increasing, which we know is not the case. Hence arises what has been termed "the struggle for existence," resulting in "the survival of the fittest." This "struggle for life" is either against the forces of inorganic or those of organic nature. Among the former are storms, floods, intense cold, long-continued droughts, or violent blizzards, all of which take toll of the weaker or less wary individuals of each species—those that are less adapted to survive such conditions. In judging how this would act, we must always remember the enormous scale on which Nature works, and that although now and then a few of the weaker individuals may live and a few of the stronger be killed, yet when we deal with hundreds of millions, of which eighty or ninety millions inevitably die every year while about ten or twenty millions only survive, it is impossible to believe that those which survive, not one year only but year after year throughout the whole existence of each species, are not on the average better adapted to the complex conditions of their environment than those which succumb to it. It is a mere truism that the _fittest survive_.