November 28, 1901] 



NA TURE 



79 



LETTERS TO THE EDITOR. 

 [ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to coi respond with the writers of, rejected 

 maniiscripts intended for this or any other part of Nature. 

 A'o notice is taien of anonvnwiis commtmications.'^ 



The Red (C) Line of Hydrogen and the Zeeman 

 Effect. 



F<iR some months we have heen engaged in an investigation 

 of the effect of a magnetic field on the more conspicuous Unes 

 of certain elementary gases, including the case of helium re- 

 ferred to by Prof. Gray and Dr. Stewart in your issue of 

 November 21. We have employed a very fine echelon grating 

 of twenty-six plates by Hilger. One observation that we have 

 already made is, perhaps, of sufficient interest to deserve 

 mention in your columns. The red (C) line of hydrogen was 

 unmistakably divided before the application of the magnetic 

 field. A reference to Michelson's papers on the application of 

 interference methods to spectroscopic measurements showed 

 that he had announced the red hydrogen line to be a very close 

 double as long ago as 18S7. A more detailed examination of 

 the visibility curve is given in the Philosophical Magazine for 

 September, 1S92, from which it appears that the curve is prac- 

 tically the same as that due to a double source, whose com- 

 ponents have the intensity ratio 7 ; 10, and in each of which the 

 light is distributed according to the exponential law resulting 

 from .Maxwell's theory of velocities. The distance between the 

 components is given as f4x io~* millim., so that it should be 

 well within the power of the echelon as at present constructed 

 to resolve the line. 



Under the influence of the magnetic field each component is 

 widened, and by using a double-image prism as recommended 

 by the late Prof. Preston to separate the constituents, is seen to 

 give rise to the normal triplet. 



It is necessary for these observations to use a vacuum tube 

 giving a bright crimson light in the capillary portion, and it is 

 an advantage to have the tube in connection with the pump and 

 a supply of hydrogen while under observation. 



We hope to be able to publish quantitative results regarding 

 this and other lines when our researches are further advanced. 



Blythswood, 

 H. S. Allen. 



Blythswood Laboratory, Renfrew, N.B., November 25. 



On the Probability that the Son of a very highly-gifted 

 Father will be no less Gifted. 



An abstract was presented last Thursday to the Royal Society 

 by Prof. Karl Pearson of results that apparently showed in a 

 most conclusive way " that the mental characters in man are in- 

 herited in precisely the same manner as the physical." His data 

 and work have yet to be communicated, but the figures, which 

 were given separately, for four physical characters in from Soo to 

 1000 pairs of brothers, and for seven mental characters in another 

 equally large set, are closely the same in all eleven instances, 

 and they seem to substantiate his conclusion up to the hilt. 



As the question of inherited ability may thus be brought 

 again to the front, perhaps you will allow me space to refute a 

 specious objection which is likely to be adduced, as it has 

 already been urged with wearisome iteration, namely, that the 

 sons of those intellectual giants whom history records, have 

 rarely equalled or surpassed their fathers. In reply, I will con- 

 fine myself to a single consideration and, ignoring what 

 Lombroso and his school might urge in explanation, will now 

 show what would be expected if these great men were as fertile 

 and as healthy as the rest of mankind. 



The objectors fail to appreciate the magnitude of the drop in 

 the scale of intelligence, from the position occupied by the 

 highly exceptional father down to the level of his genetic focus 

 (as I have called it), that is to the point from which his offspring 

 deviate, some upwards, some downwards. They do not seem 

 to understand that only those sons whose upward deviation ex- 

 ceeds the downward drop can attain to or surpass the paternal 

 level of intelligence, and how rare those wide deviations must 

 be. 



The exceptional quality of the father is only one of four 

 elements that contribute in apparently equal shares to determine 



NO. 1674, VOL. 65] 



the position of the genetic focus. The other three are (l) the 

 quality of the mother, (2) that of the paternal ancestry, (3) that 

 of the maternal ancestry. In the case we are supposing the 

 mother may occupy a high, (hough almost necessarily a lower, 

 position on the scale of intelligence than the father. Where, 

 for instance, could an intellectual giant like Napoleon find an 

 equal mate ? The average ancestry, whether of the father or of 

 the mother, are always more or less mediocre, some ancestors 

 being above and others below the general level of intelligence. 

 Consequently the exceptional quality of the father, considered 

 apart from his ancestry, is not likely to raise the'position of the joint 

 genetic focus of himself and the mother by more than a quarter 

 of its amount. Let us consider the far from overstrained case 

 of a father whose intelligence exceeds mediocrity by an amount 

 that lies between seven and eight times that of the " probable 

 error " of the distribution of racial intelligence. Extending the 

 nomenclature employed in my lecture, which you published on 

 October 31, his class would be V. I will suppose his wife to 

 be a woman of such ability that her equal is only to be found 

 once in every fifty persons, that is of class U. Then the class 

 of the mid-parent would be half-way between Y and U, or W. 

 Regression which is due to the joint ancestral influences would 

 degrade W by at least two classes, that is from W to U, which 

 makes a total drop of four classes from the Y from which we 

 started. Only those children who deviate upwards to that large 

 extent can equal their father. But the conditions are still harder 

 than they appear, because of the closeness with which the sons 

 are clustered round the common filial (or genetic) centre. 

 Their modulus of deviation is less than that of racial deviation, 

 so that it would need fully five steps of filial deviation to 

 reach the required level, and hardly one in 300 deviates do 

 that. He might have many sons more or less distinguished, 

 sons classifiable as W, X, or V, as experience shows to be 

 the case, but the probability of a Y father having a Y son is 

 remote. All the same, a Y father is more likely than any one 

 man of a lower class than his own to have such offspring, but 

 as the latter are very numerous the supply of Y men comes 

 chiefly from them. 



I have looked again at my " Hereditary Genius," written 

 many years ago, under the light of newer knowledge, and feel 

 that the evidence there recorded of the inheritance of ability is 

 quite as strong as theory would lead us to expect. 



I must not trespass further on your space, though the subject 

 tempts one to go far into details. Francis Galton. 



Pigments of Nudibranchiate Mollusca. 



Last summer, on the coast of California, I had occasion to 

 study three species of the beautiful genus Chromodoris, all of 

 them hitherto undescribed. Technical descriptions have been 

 sent for publication elsewhere, but the purpose of the present 

 note is to call attention to the interesting pigments possessed 

 by these animals. C. iiniversttatis (so called because it bears 

 the colours of the University of California) is a large species, 

 more than 2^ inches in length, of a rich dark ultramarine blue, 

 the edges of the mantle and foot bright cobalt blue. The 

 mantle has two longitudinal series of oblong very bright orange 

 spots, about seven in a series ; there are also five orange spots 

 on the anterior part of the mantle. The sides of the foot also 

 exhibit a row of orange spots. 



When the animal is placed in formalin (4 per cent.) it imme- 

 diately gives into solution a strong blue colour. This colour • 

 is even dissolved out, though more slowly, by sea water after 

 the death of the animal. The blue solution is bleached by 

 caustic potash, and is immediately turned pink (about the 

 colour of apricot flowers) by hydrochloric acid. 



The orange spots are not affected by formalin, but, curiously, 

 when seen through the blue solution, they appear bright red. 

 C. porterae (from La Jolla) is a small species, about 1 1 mm. long, 

 blue as in the first species, with two rather broad longitudinal 

 stripes of bright orange on the mantle. There is an incon- 

 spicuous median line of a lighter blue. After death the blue 

 (evidently the same pigment as that of C. universitatis) dissolves 

 out, and the body becomes a sort of pale greenish, with the 

 dorsal stripe on the mantle very white. The orange bands are 

 not affected. 



The third species, C. iiicfarlandi (from La Jolla and San 

 Pedro), is about 35 mm. long, the mantle brilliant purple with 

 a yellow margin and three longitudinal yellow stripes. The end 



