Charles Darwin: The Origin of Species by means of Natural Selection

CHAPTER IX. HYBRIDISM. 2. DEGREES OF STERILITY. (continued)

Finally, considering all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.