f^OC 



NATURE 



[November 24, 192 1 



theory of the atom ; the basis of the fundamental 

 hypothesis / pdq = nh is purely mathematical, and 



cannot be stated apart from mathematical conceptions. 

 So also, I think, does the Maxwellian theory of the 

 electromagnetic field, but to discuss this matter would 

 lead us too far. All that I am concerned to assert is 

 that there is no sense of the word "real," relevant to 

 experimental physics, in which the principle of rela- 

 tivity has a different kind of influence on our views 

 of what is real from any other theory. In particular, 

 it has no influence whatever on the belief that matter 

 is real in any scientific sense. It may have some 

 bearing on that doctrine in the metaphysical sense; 

 but since, after considerable philosophic reading, I 

 am still unable to discover what metaphysicians mean 

 by "real," I clearly cannot discuss that question. But 

 since, again, I can understand science without under- 

 standing metaphysics, I am naturally convinced that 

 the two are completely independent. 



Norman R. Campbell. 



Dr. Norman Campbell has not understood me. 

 Probably thinking that I am an idealist philosopher, 

 he has supposed that I must be arguing that there is 

 no scientific reality in the accepted meaning — that is, no 

 scientific criterion of reality — and that the naturalist's 

 mongoose, for example, has just as much or just as 

 little realitv as the drunkard's. What I was pointing 

 out was the fact that the principle of relativity is 

 the rejection of materialism. Materialism is a causal 

 theory of scientific reality. It is the argument that 

 when we pronounce anything in our sense-experience 

 to be real we imply an independent cause of it. 

 According to the principle of relativity, the inference 

 is entirelv unnecessarv and to insist on it unscientific. 

 Instead of this causal theory relativity offers a simple 

 correspondence theory. The Minkowski-Einstein uni- 

 verse consists of events co-ordinated bv observers in 

 their different svstems of reference. What is essential 

 to constitute the "real event" of any observer is 

 that there should be point-to-point correspondence 

 between his co-ordination of It and the different co- 

 ordinations of other observers. The co-ordination of 

 an event by any observer — that Is, his perspective of 

 the event — Is not an effect which Is the appearance 

 to him of a "causal" realitv, but an actual case in 

 point of the realitv itself. The "event " in the four- 

 dimensional continuum, and Its track the "world- 

 line," In re-forming the notion of scientific reality has 

 relegated scientific materialism to its right place In 

 the limbo of scholasticism. Whatever his disagree- 

 ment, at least Dr. Campbell need not be alarmed for 

 the basis of scientific research. 



November 9. H. Wildon Carr. 



Hybridity and the Evolution of Species. 



I AM sorry to say that the postscript of "The Writer 

 of the Article " to my letter on p. 274 in Nature of 

 October 27 is not according to facts. It was he who 

 used Trillium, Dirca, and Scoliopus as evidence 

 against "bad pollen " being an indication of hybridity; 

 this evidence appeared to me to be insufficient, and 

 I stated the reason why. In his postscript "The 

 Writer of the Article " makes no attempt to refute 

 my arguments against his view, but says : " In such 

 cases as Trillium, Dirca, and Scoliopus it is not 

 suflficlent for him [meaning me] to suggest that thev 

 must be hybrids merely because thev have bad pollen," 

 though I have never suggested this, or referred to 

 Trillium, Dirca, and Scoliopus in any of my previous 

 writings. 



NO. 2717, VOL. 108] 



Perhaps I may be allowed to make use of this 

 occasion to state my point of view shortly with regard 

 to the question of bad pollen. I do not think that 

 bad pollen is proof of a hybrid origin, but consider it 

 as "suspect"; neither do I share Jeffrey's view that 

 absence of bad pollen is a sign of a non-hybrid origin ; 

 as a fact, I know that it is not. Some of my segre- 

 gates of the cross Antirrhinum gluHnosum x majus 

 have bad pollen, while others have not. I further 

 think that "The Writer of the Article" is mistaken 

 in his view that the theory of mutation requires the 

 occurrence of a certain proportion of defective germ- 

 cells. The facts are these : — When de Vries found 

 bad pollen in Oenothera Lamarckiana he accounted 

 for its presence on the assumption that this defect was 

 caused by mutations ; we now know that O. 

 Lamarckiana is a hybrid, so that it is much more 

 probable that hybridity is the cause of the presence 

 of bad pollen ; recent cytological work seems even to 

 prove this. 



May I beg zoologists to answer a question I should 

 like to put to them, namely : Is there any evidence 

 that the presence of oligopyrene and apyrene sperms 

 in some insects and molluscs is due to hybridity? 



I might finish my remarks here were It not that 

 "The Writer of the Article" reproaches me with 

 "begging the question " at issue. Nothing is farther 

 removed from my Intentions, so that I desire to deal 

 shortly with all the points mentioned by him. He 

 argues that it militates against the general applic- 

 abllitv of the origin of species by hybridisation that 

 not all British roses are hybrids. I fail to see the 

 force of this argument, as it is well known that 

 homozygotes can arise from a cross without showing 

 any sign whatever of their hybrid origin ; con- 

 sequently, the fact of specific purity can never be used 

 as an argument against a hybrid origin. 



Nor does the fact that pollen sterility and fer- 

 tility behave as a pair of characters in the sweet pea 

 and the velvet bean tell against the origin of that bad 

 pollen by hybridisation, as "The Writer of the 

 Article" seems to think, unless he can bring forward 

 arguments in favour of his contention which are 

 unknown to me. Until then I must acknowledge I 

 fail to see how behaviour of a character already exist- 

 ing can reveal its mode of origin ; the idea that specific 

 characters do not segregate while varietal characters 

 do Is, of course, obsolete. 



"The Writer of the Article" finishes his remarks 

 by pointing out that It will be necessary to bring some 

 more convincing argument In support of hybridisation 

 as a constructive evolutionary factor before it Is likely 

 to receive much serious consideration from biologists. 

 If he means some more convincing argument than 

 the suggested hybrid nature of Trillium, Dirca, and 

 Scoliopus — a suggestion which is not mine — I cordially 

 agree with him. I wonder whether the following will 

 assist him In taking a kinder view of my theory than 

 he evidently does? 



It is a generally acknowledged fact that new breeds 

 of animals and plants can arise by crossing, while 

 no other mode of origin of them has ever been proved, 

 although various other modes have been suggested. 

 We all know that Darwin explained the origin of new 

 forms in Nature largely on the mode of origin _ of 

 domesticated races, so that It Is of considerable Im- 

 portance to know the real nature of the "variations" 

 among plants and animals under domestication which 

 plav so Important a r6le in Darwin's writings. 



Some time ago I happened to come across a letter 

 of Darwin himself In his "Life and Letters," which 

 seems to thrown Important light on this momentous 

 Question. The letter is printed in full on o. -^42 of 

 the third volume of the "Life and Letters "; it was 



