54 



NATURE 



[November 21, 1907 



intermittently from lime to time, but it was not enough 

 to move tlie ammeter needle, and was most likely due to 

 a further slow evolution of gas from the still heated 

 surfaces. A current could be passed by a coil from the 

 hot kathode to a third electrode as anode without causing 

 any appreciable resumption of the flow in the 250-volt 

 circuit. 



More gas was then generated within the apparatus by 

 heating the third electrode with the coil discharge, and 

 the current in the main circuit resumed its original in- 

 tensity, again heating up the calcium anode. The original 

 phenomenon was repeated, a sudden cessation of current 

 raking place when the calcium volatilised. Just before 

 stopping, the glow of the tube changed to that character- 

 istic of argon, so probably a trace of air had not been 

 removed. The whole phenomenon could be repeated by 

 admitting oxygen to the apparatus and proceeding as 

 before. 



This experiment shows that in a sufificiently high vacuum 

 the Wehncit electrode ceases to be effective. In the ex- 

 periments so far recorded the saturation current has 

 increased with the improvement of the vacuum, and the 

 phenomenon has been supposed to be in the first place 

 independent of the residual trace of gas present. Wehnelt 

 {loc. cit., p. 445) remarks : — " Fiir Drucke unter o-i mm. 

 ist die fiir eine bestimmte Temperatur ausgesandte Zahl 

 von negativen lonen unabhangig vom Druck," and 

 (p. 456) " die Grenzstromstarken um so hbher sein . . . 

 je tiefer die Druck ist." In his description of his modifi- 

 cation of the Braun tube (Phys. Zeit., 1905, vi., 732) he 

 says the vacuum in the tube must be as perfect as possible. 



Richardson, whose mathematical theory of the general 

 phenomenon has received quantitative experimental con- 

 firmation, and has been accepted by Wehnelt in the case 

 of his electrode, regarded the action as purely electronic. 

 Commenting on the magnitude of the current and the 

 smallness of the residual gas — in one experiment 2 amperes 

 per sq. cm. at a measured pressure of o-ooi6 mm. from 

 a carbon lamp-filament — he says (Joe. cit., p. 546) : — 

 " This (the current) is twenty-five times the ma.ximum value 

 obtained by supposing each molecule to produce one ion ; 

 so that it is highly improbable that any considerable part 

 of the conductivity investigated is due to ions produced in 

 this way. . . . Both these points of view lead to the con- 

 clusion that the corpuscles are not produced by a dynamical 

 action between the molecules of the surrounding gas and 

 the surface of the metal. In fact, all the experimental 

 results seem to point to the view that the corpuscles are 

 produced from the metal by a process similar to evapor- 

 ation." 



These isolated quotations, of course, may not fairly 

 express the opinions of the authors about what is a very 

 complex phenomenon ; but the general impression their 

 results has conveyed, I think, has been that the large 

 currents dealt with were wholly conveyed by the expelled 

 electrons, and therefore should pass through any vacuum, 

 however perfect. I do not think the electronic emission 

 can account for more than a negligible fraction of the 

 total current, which is carried almost wholly by the 

 residual gas. 



The results here given bear out the general view I have 

 from time to time advocated since my experience with 

 the use of calcium, that degrees of vacuum are in practice 

 apt to be overrated, and really high degrees of vacuum 

 arc not so readily obtained as is commonly supposed. 



Frederick Soddv. 



Physical Chemistry Laboratory, The University, 

 Glasgow. 



The Interpretation of Mendelian Phenomena. 

 I AM Strongly inclined to agree with Dr. ."Xrchdall Reid 

 that Mendelian investigations throw no light on many 

 of the most important problems of biology, such as the 

 causes of variation, the evolution of adaptations, and many 

 others. On the other hand, it is difficult to understand 

 what Dr. Archdall Reid means by the statement that 

 Mendelism is the investigation of sex. In one of his 

 letters he describes Mendelian phenomena as " abnormali- 

 ties of sexual reproduction which occur under conditions 

 of artificial selection." If this means peculiarities of 



NO. 1986. VOL. yyl 



heredity in sexual reproduction observed in cultivated 

 varieties, it may be allowed to pass ; but in an appendix 

 to the second edition of his " Principles of Heredity," Dr. 

 Reid states that the inheritance of Mendelian characters 

 is probably sexual. He proceeds as follows ; — " Nature 

 has evolved alternative inheritance to create and per- 

 petuate sexual differentiation, but, just as blending of 

 sexual characters sometimes occurs, so on the other hand 

 the inheritance of non-sexual characters is sometimes 

 alternative. As we have seen, whenever the latter 

 happens, the non-se.xual differences are, like the sexual 

 differences, usually considerable. Nature makes the mis- 

 take, so to speak, of treating them as se.xual. Of course, 

 however, the alternative inheritance of large non-sexual 

 differences is not certain, not so ' clean ' as that of sexual 

 differences, the alternative inheritance of which has been 

 established by a long course of selection." 



According to this, the alternative inheritance of non- 

 sexual characters is of the same nature as sexual 

 dimorphism. It seems to me that Dr. Reid has not 

 sufficiently studied the inheritance of secondary sexual 

 characters. It is admitted by almost all evolutionists from 

 Darwin himself downwards that selection cannot have 

 been the cause of the alternative inheritance of sexua! 

 differences. The female in selecting a particular male 

 cannot prevent the transmission of his peculiarities to her 

 daughters. The male characters must be limited to the 

 male sex from their first origin as variations, because 

 otherwise the selection of the individual male would merely 

 ensure their transmission to both sexes. 



The essential peculiarity of secondary sexual characters 

 is their physiological connection with the primary, i.e. 

 with the male or female gonads. It is not merely a 

 question of alternative inheritance ; both male and female 

 characters are inherited by each individual, but normally 

 only one set is developed. When, however, the male 

 gonads are removed, the male characters are usually not 

 normally developed, but suppressed. In Mendelian cases 

 the development of alternative characters is usually entirely 

 independent, both in theory and fact, of the sex or con- 

 dition of the gonads. 



The contrast of male and female corresponds to 

 alternative dominance in a heterozygote ; male characters 

 are dominant in male, female in female, but either can 

 and does transmit both. In Mendelian cases alternative 

 inheritance is segregative ; a recessive transmits only 

 recessive characters, a pure dominant only the dominant. 

 In other passages which I could quote Dr. Reid has shown 

 that he has failed to appreciate this fundamental distinc- 

 tion between dominance and segregation, between the 

 heterozygote and the homozygote, between the first gener- 

 ation of a Mendelian cross and the second. 



Excepting parthenogenesis, the heredity of all characters 

 is sexual in that it is connected with fertilisation, but 

 I do not see that Mendelian characters are sexual in any 

 other sense. The theory that the inheritance of sexual 

 characters is Mendelian is one which has a definite mean- 

 ing and can be investigated. Dr. Archdall Reid's state- 

 ment that Mendelian inheritance is sexual is in one sense 

 a truism, in another seems to me to have no real meaning 

 at all, for to say that nature has evolved alternative 

 inheritance to perpetuate sexual differentiation explains 

 nothing. J. T. Cinningham. 



llighgate, November 3. 



I .\M very willing, and, like most people with theories, 

 I believe I am able to maintain the correctness of the 

 views to which Mr. Cunningham refers ; but to discuss 

 my speculations now would be to confuse the issue. I 

 may say, however, that the appendix to the second edition 

 of my work, from which Mr. Cunningham quotes, is a 

 mere sketch hastily thrown together to meet the objec- 

 tions of critics who had advised the lay and scientific 

 public that a book, which I fondly hoped contained a little 

 that was new, and which certainly contained more than 

 a little of which Mendelians seem profoundly unaware, 

 was antiquated and worthless, not because there were no 

 new facts or inferences in it, nor because its facts and 

 inferences were invalid, but simply and solely because I 

 had not adopted " the new^ method " nor accepted the 



