266 



CAUSES OF VARIABILITY. 



Chap. XXII. 



of distinct species, besides commingling their characters, adds greatly to 

 their variability, it has probably arisen that some botanists have gone so 

 far as to maintain 39 that, when a genus includes only a single species, this 

 when cultivated never varies. The proposition made so broadly cannot 

 be admitted ; but it is probably true that the variability of cultivated 

 monotypic genera is much less than that of genera including nume- 

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

 I have stated in my ' Origin of Species/ and in a future work shall more 

 fully show, that the species belonging to small genera generally yield a less 

 number of varieties in a state of nature than those belonging to lame 

 genera. Hence the species of small genera would, it is probable, produce 



under 



:enera. 



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 distinguishing, 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 Icevis 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 resembled 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 exaggeration 

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

 comes 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 



_ 



39 This was the opinion of the elder 1837, p. 37, has discussed this same 



De Candolle, as quoted in ' Die. Class. point. 



d'Hist. Nat./ torn. viii. p. 405. Puvis, *o < Comptes Kendus,' Novembre 21, 



in his work, < De la Degeneration,' 1864, p. 838. 



* 





