On the theory of natural selection the case is especially important, in a such as the sterility of hybrids could not possibly be of any advantage to them, and could not have been acquired by the continued preservation of successive profitable degrees of sterility. Hybrids have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs themselves are perfect in structure, as far as the microscope reveals. The fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is the fertility of pure species. The hybrids, moreover, produced from reciprocal crosses often differ in fertility. The slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention.