August 25, 1904] 



NA TURE 



413 



pure, is taking place. The significance and origin of the 

 discontinuity of variation is therefore in great measure 

 evident. So far as pre-existing elements are concerned, it 

 is an expression of the power of cell-division to distribute 

 character-units among gametes. The initial purity of so 

 many nascent mutations is thus no longer surprising, and, 

 indeed, that such initial purity has not been more generally 

 observed we may safely ascribe to imperfections of method. 



It is evident that the resemblance between the parent 

 originating a variety and a heterozygote is close, and the 

 cases need the utmost care in discrimination. If, for 

 instance, we knew nothing more of the Andalusian fowl 

 than that it throws blacks, blues, and whites, how should 

 we decide whether the case was one of heterozygosis or of 

 nascent mutation? The second (F,) generation from Brown 

 I-eghorn X White Leghorn contains an occasional Silver- 

 <^irey or Duckwing female. Is this a mutation induced by 

 crossing, or is it simply due to a recombination of pre- 

 existing characters? We cannot yet point to a criterion 

 which will certainly separate the one from the other ; but 

 perhaps the statistical irregularity usually accompanying 

 mutation, contrasted with the numerical symmetry of the 

 gametes after normal heterozygosis, may give indications 

 in simple cases — though scarcely trustworthy even there. 

 These difficulties reach their maximum in the case of types 

 which are continually giving off a second form with greater 

 or less frequency as a concomitant of their ordinary exist- 

 ence. This extraordinarily interesting phenomenon, pointed 

 out first by De Vries, and described by him under the head 

 of " Halb- " and " Mittel-Rassen," is too imperfectly under- 

 stood for me to do more than refer to it, but in the attempt 

 to discover what is actually taking place in variation it 

 must play a considerable part. 



Just as that normal truth to type, which we call heredity, 

 is in its simplest elements only an expression of that 

 qualitative symmetry characteristic of all non-differentiating 

 cell-divisions, so is genetic variation the expression of a 

 qualitative asymmetry beginning in gametogenesis. Vari- 

 ation is a novel cell-division.' So soon as this fact is 

 grasped we shall hear no more of heredity and variation 

 as opposing " factors " or " forces " — a metaphor which 

 has too long plagued us. 



We cease, then, to wonder at the suddenness with which 

 striking variations arise. Those familiar with the older 

 literature relating to domesticated animals and plants will 

 recall abundant instances of the great varieties appearing 

 early in the history of a race, while the finer shades had 

 long to be waited for. In the sweet pea the old purple, 

 the red bicolor, and the white have existed for gener- 

 ations, appearing soon after the cultivation of the species; 

 but the finer splitting which gave us the blues, pinks, &'c. , 

 is a much rarer event, and for the most part only came 

 when crossing was systematically undertaken. If any of 

 these had been seen before by horticulturists, we can feel 

 no doubt whatever they would have been saved. An 

 observer contemplating a full collection of modern sweet 

 peas, and ignorant of their history, might suppose that the 

 extreme types had resulted from selective and more or less 

 continuous intensification of these intermediates, exactly 

 inverting the truth. 



We shall recognise among the character-groups lines of 

 cleavage, along which they easily divide, and other finer 

 subdivisions harder to effect. Rightly considered, the sudden 

 appearance of a total albino or a bicolor should surprise us 

 less than the fact that the finer shades can appear at all. 



.At this point comes the inevitable question, what makes 

 the character-group split? Crossing, we know, may do 

 this : but if there be no crossing, what is the cause of 

 variation? With this question we come sharply on the edge 

 of human knowledge. But certain it is that if causes of 

 variation are to be found by penetration, they must be 

 -specific causes. A mad dog is not " caused " by July heat, 

 nor a moss rose by progressive culture. We await our 

 Pasteur ; founding our hope of progress on the aphorism of 

 \'irchow, that every variation from type is due to a patho- 

 logical accident, the true corollary of " Oinnis cellula e 

 cclhila." 



^ The par.illel between the differentiating divisions by which the parts of 

 the normal body are segregated from each other, and the segregating pro- 

 cesses ofgametogenesis, must be very close. Occasionally we even see the 

 -segregation of Mendelian ch.iracters among zygotic cells. 



NO. 18 I 7, VOL. 70] 



In imperfect fashion I have now sketched the lines by 

 which the investigation of heredity is proceeding, and some 

 of the definite results achieved. We are asked sometimes, 

 Is this new knowledge of any use? That is a question with 

 which we, here, have fortunately no direct concern. Our 

 business in life is to find things out, and we do not look 

 beyond. But as regards heredity, the answer to this 

 question of use is so plain that we may give it without 

 turning from the way. 



We may truly say, for example, that even our present 

 knowledge of heredity, limited as it is, will be found of 

 extraordinary use. Though only a beginning ha? been 

 made, the powers of the breeder of plants and animals are 

 vastly increased. Breeding is the greatest industry to which 

 science has never yet been applied. This strange anomaly 

 is over ; and, so far at least as fixation or purification of 

 types is concerned, the breeder of plants and animals may 

 henceforth guide his operations with a great measure of 

 certainty. 



There are others who look to the science of heredity with 

 a loftier aspiration ; who ask. Can any of this be used to 

 help those who come after to be better than we are — 

 healthier, wiser, or more worthy? The answer depends on 

 the meaning of the question. On the one hand it is certain 

 that a competent breeder, endowed with full powers, by 

 the aid even of our present knowledge, could in a few 

 generations breed out several of the morbid diatheses. As 

 we have got rid of rabies and pleuro-pneumonia so we 

 could exterminate the simpler vices. Voltaire's cry, 

 "Ecraser I'infdme! " might well replace Archbishop Parker's 

 Table of Forbidden Degrees, which is all the instruction 

 Parliament has so far provided. Similarly, a race may 

 conceivably be bred true to some physical and intellectual 

 characters considered good. The positive side of the 

 problem is less hopeful, but the various species of mankind 

 offer ample material. In this sense science already suggests 

 the way. \o one, however, proposes to take it ; and so 

 long as, in our actual laws of breeding, superstition remains 

 the guide of nations, rising ever fresh and unhurt from the 

 assaults of knowledge, there is nothing to hope or to fear 

 from these sciences. 



But if, as is usual, the philanthropist is seeking for some 

 external application by which to ameliorate the course of 

 descent, knowledge of heredity cannot help him. The 

 answer to his question is No, almost without qualification. 

 We have no experience of any means by which transmission 

 may be made to deviate from its course ; nor from the 

 moment of fertilisation can teaching, or hygiene, or exhort- 

 ation pick out the particles of evil in that zygote, or put 

 in one particle of good. From seeds in the same pod may 

 come sweet peas climbing five feet high, while their own 

 brothers lie prone upon the ground. The stick will not 

 make the dwarf peas climb, though without it the tall can 

 never rise. Education, sanitation, and the rest, are but the 

 giving or withholding of opportunity. Though in the 

 matter of heredity every other conclusion "has been 

 questioned, I rejoice that in this we are all agreed. 



NOTES. 



The sum of 120Z. has been granted by the Paris Municipal 

 Council to Prof. Grancher in furtherance of his researches 

 as to the means of preventing tuberculosis in schools. 



Os Monday and Tuesday, September 12 and 13, a visit 

 is to be paid to London by a large party of Belgian 

 engineers, members of the Association des ing^nieurs sortis 

 de I'Ecole de Li^ge — one of the most important technical 

 societies on the Continent. The party will be the guests 

 of the Iron and Steel Institute. 



Prof. Appell, dean of the faculty of sciences in the 

 University of Paris, has had the civil title of commandeur 

 de la Legion d'honneur conferred upon him by the French 

 Minister of War. Prof. Appell has served for some time 

 on the commission appointed to examine inventions likely 

 to be of service to the French Army and Navy. 



