250 BOTANY [Chap. X 



Letter 583 regretted, and I am glad to see that it was one of A. De Can- 

 dolle's desiderata. By the way, he is curiously contradictory 

 on subject. I am far from expecting that no cases of apparent 

 impossibility will be found ; but certainly I expect that ulti- 

 mately they will disappear ; for instance, Campanulaceas seems 

 a strong case, but now it is pretty clear that they must be 

 liable to crossing. Sweet-peas, 1 bee-orchis, and perhaps 

 hollyhocks are, at present, my greatest difficulties ; and I find 

 I cannot experimentise by castrating sweet-peas, without 

 doing fatal injury. Formerly I felt most interest on this 

 point as one chief means of eliminating varieties ; but I feel 

 interest now in other ways. One general fact [that] makes 

 me believe in my doctrine, 2 is that no terrestrial animal in 

 which semen is liquid is hermaphrodite except with mutual 

 copulation ; in terrestrial plants in which the semen is dry 

 there are many hermaphrodites. Indeed, I do wish I lived at 

 Kew, or at least so that I could see you oftener. To return 

 again to subject of crossing : I have been inclined to speculate 

 so far, as to think (my ! ?) notion (I say my notion, but I think 

 others have put, forward nearly or quite similar ideas) perhaps 

 explains the frequent separation of the sexes in trees, which I 

 think I have heard remarked (and in looking over the mono- 

 and dioecious Linnean classes in Persoon seems true) are very 

 apt to have sexes separated ; for [in] a tree having a vast 

 number of flowers on the same individual, or at least the same 

 stock, each flower, if only hermaphrodite on the common plan, 

 would generally get its own pollen or only pollen from another 

 flower on same stock, — whereas if the sexes were separate 

 there would be a better chance of occasional pollen from 

 another distinct stock. I have thought of testing this in your 

 New Zealand Flora, but I have no standard of comparison, 

 and I found myself bothered by bushes. I should propound 



1 In Lathyrus odoratus the absence of the proper insect has been 

 supposed to prevent crossing'. See Variation under Domestication, 

 Ed. 11., Vol. II., p. 68 ; but the explanation there given for Pisum may 

 probably apply to Lathyrus, 



2 The doctrine which has been epitomised as Nature abhors perpetual 

 self-fertilisation, and is generally known as Knight's Law or the Knight- 

 Darwin Law, is discussed by Francis Darwin in Nature, 1898. References 

 are there given to the chief passages in the Origin of Species, etc., bearing 

 on the question. See Letter 19, Vol. I. 



