The facts above given refer to undoubted varieties, excepting the trees crossed by Clotzsch, which are ranked by various botanists as strongly-marked races, sub-species, or species. That true hybrids raised from entirely distinct species, though they lose in fertility, often gain in size and constitutional vigour, is certain. It would be superfluous to quote any facts; for all experimenters, Koelreuter, Gaertner, Herbert, Sageret, Lecoq, and Naudin, have been struck with the wonderful vigour, height, size, tenacity of life, precocity, and hardiness of their hybrid productions. Gaertner[296] sums up his conviction on this head in the strongest terms. Koelreuter[297] gives numerous precise measurements of the weight and height of his hybrids in comparison with measurements of both parent-forms; and speaks with astonishment of their "_statura portentosa_," their "_ambitus vastissimus ac altitudo valde conspicua_." Some exceptions to the rule in the case of very sterile hybrids have, however, been noticed by Gaertner and {131} Herbert; but the most striking exceptions are given by Max Wichura,[298] who found that hybrid willows were generally tender in constitution, dwarf, and short-lived. Koelreuter explains the vast increase in the size of the roots, stems, &c., of his hybrids, as the result of a sort of compensation due to their sterility, in the same way as many emasculated animals are larger than the perfect males. This view seems at first sight extremely probable, and has been accepted by various authors;[299] but Gaertner[300] has well remarked that there is much difficulty in fully admitting it; for with many hybrids there is no parallelism between the degree of their sterility and their increased size and vigour. The most striking instances of luxuriant growth have been observed with hybrids which were not sterile in any extreme degree. In the genus Mirabilis, certain hybrids are unusually fertile, and their extraordinary luxuriance of growth, together with their enormous roots,[301] have been transmitted to their progeny. The increased size of the hybrids produced between the fowl and pheasant, and between the distinct species of pheasants, has been already noticed. The result in all cases is probably in part due to the saving of nutriment and vital force through the sexual organs not acting, or acting imperfectly, but more especially to the general law of good being derived from a cross. For it deserves especial attention that mongrel animals and plants, which are so far from being sterile that their fertility is often actually augmented, have, as previously shown, their size, hardiness, and constitutional vigour generally increased. It is not a little remarkable that an accession of vigour and size should thus arise under