442 GENERAL RESULTS. Chap. XIL 



conditions; and this seems not improbable, judging 

 from the remarkable effects of changed conditions on 

 the reproductive system of Abutilon and Eschscholtzia. 



Some few plants, owing to their structure, for in- 

 stance, Ophrys apifera, have almost certainly been 

 propagated in a state of nature for thousands of 

 generations without having been once intercrossed ; 

 and whether they would profit by a cross with a fresh 

 stock is not known. But such cases ought not to make 

 us doubt that as a general rule crossing is beneficial 

 and self-fertilisation injurious, any more than the 

 existence of plants which, in a state of nature are 

 propagated asexually, that is, exclusively by rhizomes, 

 stolons, &c* (their flowers never producing seeds), 

 should make us doubt that seminal generation must 

 have some great advantage, as it is the common plan 

 followed by nature. Whether any species has been 

 reproduced asexually from a very remote period cannot, 

 of course, be ascertained. Our sole means for forming 

 any judgment on this head is the duration of the 

 varieties of our fruit trees which have been long pro- 

 pagated by grafts or buds. Andrew Knight formerly 

 maintained that under these circumstances they always 

 become weakly, but this conclusion has been warmly 

 disputed by others. A recent and competent judge, 

 Prof. Asa Gray,f leans to the side of Andrew Knight, 

 which seems to me, from such evidence as I have 

 been able to collect, the more probable view, notwith- 

 standing many opposed facts. 



With respect to the first of the two propositions 

 at the head of this chapter, namely, that cross-fertilisa- 

 tion is generally beneficial, we have excellent evidence. 



* I have given several cases in t ' Darwiniana : Essays and 



my 'Variation under Domestica- Reviews pertaining to Darwinism/ 



tion,' ch. xviii. 2nd edit. vol. ii. 1876, p. 338 

 p. 152. 



