September 21, 191 1] 



NATURE 



405 



1 have been asked by an Australian breeder whether it is 

 possible to combine the optimum length of wool with the 

 optimum fineness and the right degree of crimping. I have 

 to reply that absolutely nothing is yet known for certain 

 .is to the physiological factors determining the length or 

 the fineness of wool. The crimping of the fibres is an 

 expression of the fact that each particular hair is curved, 

 and if free and untwisted would form a corkscrew spiral, 

 but as to the genetics of curly hair even in man very little 

 is yet known. But leaving the question of curl on one 

 side, we have, in regard to the length and fineness of wool, 

 a problem which genetic experiment ought to be able to 

 solve. Note that in it, as in almost all problems of the 

 " yield " of any product of farm or garden, two distinct 

 elements are concerned — the one is size, and the other is 

 number. The length of the hair is determined by the rate 

 of excretion and length of the period of activity of the 

 hair follicles, but the fineness is determined by the number 

 of follicles in unit area. Now analogy is never a safe 

 l»uide, but I think if we had before us the results of really 

 critical experiments on the genetics of size and number 

 of multiple organs in any animal or even any plant, we 

 night not wholly be at a loss in dealing with this important 

 problem. 



A somewhat similar question comes from South Africa. 

 Is it possible to combine the qualities of a strain of ostriches 

 which has extra long plumes with those of another strain 

 which has its plumes extra lustrous? I have not been able 

 fully to satisfy myself upon what the lustre depends, but I 

 incline to think it is an expression of fineness of fibre, 

 which again is probably a consequence of the smallness and 

 increased number of the excreting cells, somewhat as the 

 fineness of wool is a consequence of the increased number 

 and smallness of the excreting follicles. 



Again the question arises in regard to flax, how should 

 a strain be bred which shall combine the maximum length 

 with maximum fineness of fibre? The element of number 

 comes in here, not merely with regard to the number of 

 fibres in a stem, but also in two other considerations : first, 

 that the plant should not tiller at the base, and, secondly, 

 that the decussation of the flowering branches should be 

 postponed to the highest possible level. 



Now in this problem of the flax, and not impossibly in 

 the others I have named, we have questions which can in 

 all likelihood be solved in a form which will be of general, 

 if not of universal, application to a host of other cognate 

 questions. By good luck the required type of flax may be 

 struck at once, in which case it may be fixed by ordinary 

 Mendelian analysis, but if the problem is investigated by 

 accurate methods on a large scale, the results may show 

 the way into some of those general problems of size and 

 number which make a great part of the fundamental 

 mystery of growth. 



I see no reason why these things should remain inscrut- 

 able. There is indeed a little light already. We are well 

 acquainted with a few examples in which the genetic 

 behaviour of these properties is fairly definite. We have 

 examples in which, when two varieties differing in number 

 of divisions are crossed, the lower number dominates — or, 

 in other words, that the increased number is a consequence 

 of the removal of a factor which prevents or inhibits 

 particular divisions, so that thev do not take place. It is 

 likely that in so far as the increased productivity of a 

 domesticated form as compared with its wild original 

 depends on more frequent division, the increase is due to 

 loss of inhibiting factors. How far may this reasoning be 

 extended? Again, we know that in several plants — peas, 

 sweet peas, Antirrhinum, and certain wheats — a tall 

 variety differs in thai respect from a dwarf in possessing 

 one more factor. It would be an extraordinarily valuable 

 addition to knowledge if we could ascertain exactly how 

 this factor operates, how much of its action is due to linear 

 repetition, and how much to actual extension of individual 

 parts. The analysis of the plants of intermediate size has 

 never been properly attempted, but would be full of interest 

 and have innumerable bearings on other cases in animals 

 and plants, some of much economic importance. 



That in all such examples the objective phenomena we 



see are primarily the consequence of the interaction of 



genetic factors is almost certain. The lay mind is at first 



disposed, as always, to attribute such distinctions to any- 



NO. 2l86. VOL. 87] 



thing rather than to a specific cause which is invisible. 

 An appeal to differences in conditions — which a moment's 

 reflection shows to be either imaginary or altogether inde- 

 pendent — or to those vague influences invoked under the 

 name of Selection, silently postponing any laborious analysis 

 of the nature of the material selected, repels curiosity for 

 a time, and is lifted as a veil before the actual phenomena ; 

 and so even critical intelligences may for an indefinite time 

 be satisfied that there is no specific problem to be investi- 

 gated, in the same facile way that, till a few years ago, 

 we were all content with the belief that malarial fevers 

 could be referred to any damp exhalations in the atmo- 

 sphere, or that in suppuration the body was discharging its 

 natural humours. In the economics of breeding, a thousand 

 such phenomena are similarly waiting for analysis and 

 reference to their specific causes. What, for instance, is 

 self-sterility? The phenomenon is very widely spread 

 among plants, and is far commoner than most people 

 suppose who have not specially looked for it. Why is it 

 that the pollen of an individual in these plants fails to 

 fertilise the ova of the same individual? Asexual multi- 

 plication seems in no way to affect the case. The 

 American experimenters are doubtless right in attributing 

 the failure of large plantations of a single variety of apples 

 or of pears in a high degree to this cause. Sometimes, as 

 Mr. W. O. Backhouse has found in his work on plums at 

 the John Innes Horticultural Institution, the behaviour of 

 the varieties is most definite and specific. He carefully 

 self-fertilised a number of varieties, excluding casual 

 pollination, and found that while some sorts — for example, 

 Victoria, Czar, and Early Transparent — set practically every 

 fruit self-pollinated, others, including several (perhaps all) 

 Greengages, Early Orleans, and Sultan, do not set a single 

 fruit without pollination from some other variety. Dr. 

 Erwin Baur has found indications that self-sterility in 

 Antirrhinum may be a Mendelian recessive, but whether 

 this important suggestion be confirmed or not, the subject 

 is worth the most minute study in all its bearings. The 

 treatment of this problem well illustrates the proper scope 

 of an applied science. The economic value of an exact 

 determination of the empirical facts is ■ obvious, but it 

 should be the ambition of anyone engaging in such a 

 research to penetrate further. If we can grasp the 

 rationale of self-sterility we open a new chapter in the 

 study of life. It may contain the solution of the question 

 What is an individual? — no mere metaphysical conundrum, 

 but a physiological problem of fundamental significance. 



What, again, is the meaning of that wonderful increase 

 in size or in " yield " which so often follows on a first 

 cross? We are no longer content, as Victorian teleology 

 was, to call it a " beneficial " effect and pass on. The 

 fact has long been known and made use of in breeding 

 stock for the meat market, and of late years the practice 

 has also been introduced in raising table poultry. Mr. 

 G. N. Collins, 1 of the U.S. Department of Agriculture, has 

 recently proposed with much reason that it might be applied 

 in the case of maize. The cross is easy to make on a 

 commercial scale, and the gain in yield is striking, the 

 increase ranging as high as qs per cent. These figures 

 sound extravagant, but from what I have frequently seen 

 in peas and sweet peas, I am prepared for even greater 

 increase. But what is this increase? How much of it is 

 due to change in number of parts, how much to trans- 

 ference of differentiation or homceosis, as I have called it — 

 leaf-buds becoming flower-buds, for instance — and how 

 much to actual increase in size of parts? To answer these 

 questions would be to make an addition to human know- 

 ledge of incalculably great significance. 



Then we have the further question, How and why does 

 the increase disapoear in subsequent generations? The 

 very uniformity of the cross-breds between pure strains 

 must be taken as an indication that the phenomenon is 

 orderly. Its subsidence is probably orderly also. Shull has 

 advocated the most natural view that heterozygosis is the 

 exciting cause, and that with the gradual return to the 

 homozygous state the effects pass off. I quite think this 

 may be a part of the explanation, but I feel difficulties, 

 which need not here be detailed, in accepting this as a 

 complete account. Some of the effect we may probably 

 also attribute to the combination of complementary factors ; 

 1 Bureau of Plant Industry, Bulletin No. 101, 1910. 



