254 CAUSES OF VARIABILITY. Chap. XXII. 



vated is generally less than that of genera including numerous 

 species, and this quite independently of the effects of crossing. 

 I have shown in my ' Origin of Species; that the species belonging 

 to small genera generally yield a less number of varieties in a state 

 of nature than those belonging to large genera. Hence the species 

 of small genera would, it is probable, produce fewer varieties under 

 cultivation than the already variable species of larger genera. 



Although we have not at present sufficient evidence that the 

 crossing of species, which have never been cultivated, leads to the 

 appearance of new characters, this apparently does occur with 

 species which have been already rendered in some degree variable 

 through cultivation. Hence crossing, like any other change in the 

 conditions of life, seems to be an element, probably a potent one, in 

 causing variability. But we seldom have the means of distinguish- 

 ing, as previously remarked, between . the appearance of really new- 

 characters and the reappearance of long-lost characters, evoked 

 through the act of crossing. I will give an instance of the difficulty 

 in distinguishing such cases. The species of Datura may be divided 

 into two sections, those having white flowers with green stems, 

 and those having purple flowers with brown stems : now Naudin 40 

 crossed Datura laivis and ferox, both of which belong to the white 

 section, and raised from them 205 hybrids. Of these hybrids, every 

 one had brown stems and bore purple flowers; so that they re- 

 sembled the species of the other section of the genus, and not their 

 own two parents. Naudin was so much astonished at this fact, 

 that he was led carefully to observe both parent-species, and he 

 discovered that the pure seedlings of D, ferox, immediately after 

 germination, had dark purple stems, extending from the young roots 

 up to the cotyledons, and that this tint remained ever afterwards 

 as a ring round the base of the stem of the plant when old. Now I 

 have shown in the thirteenth chapter that the retention or exaggera- 

 tion of an early character is so intimately related to reversion, that it 

 evidently conies under the same principle. Hence probably we 

 ought to look at the purple flowers and brown stems of these 

 hybrids, not as new characters due to variability, but as a return 

 to the former state of some ancient progenitor. 



Independently of the appearance of new characters from crossing, 

 a few words may be added to what has been said in former chapters 

 on the unequal combination and transmission of the characters 

 proper to the two parent- forms. When two species or races are 

 crossed, the offspring of the first generation are generally uniform, 

 but those subsequently produced display an almost infinite diversity 

 of character. He who wishes, says Kolreuter, 41 to obtain an endless 

 number of varieties from hybrids should cross and recross them. 

 There is also much variability when hybrids or mongrels ar« 

 reduced or absorbed by repeated crosses with either pure parent* 



40 'ComptesRendus/Novembre 21, 41 'Nova Acta, St. Petersburg, 



1864, p. 838. 1794, p. 391. 



