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, Kolreuter, Gartner, Herbert, Sageret, Lecoq, and Naudin, have been struck with the wonderful vigour, height, size, tenacity of life, precocity, and hardiness of their hybrid productions. Gartner (17/58. Gartner 'Bastarderzeugung' s. 259, 518, 526 et seq.) sums up his conviction on this head in the strongest terms. Kolreuter (17/59. 'Fortsetzung' 1763 s. 29; 'Dritte Fortsetzung' s. 44, 96; 'Act. Acad. St. Petersburg' 1782 part 2 page 251; 'Nova Acta' 1793 pages 391, 394; 'Nova Acta' 1795 pages 316, 323.) gives numerous precise measurements of the weight and height of his hybrids in his 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 Gartner and Herbert; but the most striking exceptions are given by Max Wichura (17/60. 'Die Bastardbefruchtung' etc. 1865 s. 31, 41, 42.) who found that hybrid willows were generally tender in constitution, dwarf, and short-lived. Kolreuter explains the vast increase in the size of the roots, stems, etc., 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 (17/61. Max Wichura fully accepts this view ('Bastardbefruchtung' s. 43), as does the Rev. M.J. Berkeley in 'Journal of Hort. Soc.' January 1866 page 70.); but Gartner (17/62. 'Bastarderzeugung' s. 394, 526, 528.) 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 (17/63. Kolreuter 'Nova Acta' 1795 page 316.) have been transmitted to their progeny. The result in all cases is probably in part due to the saving of nutriment and vital force through the sexual organs acting imperfectly or not at all, 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