412 



NA TURE 



[August 25, 1904 



determine the laws governing the distribution of characters 

 in the cell-divisions of gametogenesis. 



There are, of course, many cases which still bafHe our 

 attempts at such analysis, but some of the most paradoxical 

 exceptions have been reduced to order by the accumulation 

 of facts. The consequences of heterozygosis are curiously 

 specific, and each needs separate investigation. A remark- 

 able case occurred in stocks, showing the need for caution 

 in dealing with contradictory results. Hoary leaves and 

 glabrous leaves are a pair of allelomorphic characters. 

 When glabrous races were crossed with crossbreds, some- 

 times the results agreed with simple e.xpectation, while in 

 other cases the offspring were all hoary when, in accordance 

 with similar e.xpectation, this should be impossible. By 

 further experiment, however. Miss Saunders has found that 

 certain glabrous races crossed together give nothing but 

 hoary heterozygotes, which completely elucidates such 

 e.xceptions. There is every likelihood that wherever segre- 

 gation occurs similar analysis will be successful. 



Speaking generally, in every case the first point to be 

 worked out is the magnitude of the character-units recog- 

 nised by the critical cell-divisions of gametogenesis, and 

 the second is the specific consequence of all the possible 

 combinations between them. When this has been done for 

 a comprehensive series of types and characters, it will be 

 time to attempt further generalisation, and perhaps to look 

 for light on that fundamental physiological property, the 

 power of cell-division. 



Segregation and Sex. — .Acquaintance with Mendelian 

 phenomena irresistibly suggests the question whether in all 

 cases of families composed of distinct types the distinctness 

 may not be primarily due to gametic segregation. Of all 

 such distinctions none is so universal or so widespread as 

 that of sex : may it not be possible that sex is due to a 

 segregation occurring between gametes, either male, female, 

 or both? It will be known to you that several naturalists 

 have been led by various roads to incline to this view. We 

 still await the proof of crucial experiments ; but without 

 taking you over more familiar ground, it may be useful 

 to show how the matter looks from our standpoint. As 

 regards actual experiment, all results thus far are compli- 

 cated by the occurrence of some sterility in the hybrid 

 generation. Correns, fertilising Q Bryonia dioica with 

 pollen from ^ B. alba, obtained offspring (F,) either cJ 

 or Q , with only one doubtful exception. Gartner found a 

 similar result in Lychnis diurna Q x ^ L. Flos-cuculi 

 as fT, but only raised six plants (4 cJ, 2 Q). From 

 L. diurna Q X S Silene noctiflora as c? he got only two 

 plants, spoken of as females which developed occasional 

 anthers. These results give a distinct suggestion that sex 

 may be determined by differentiation among the male 

 gametes, but satisfactory and direct proofs can only be 

 obtained from some case where sterility does not ensue. 



Apart, however, from such decisive evidence — which, 

 indeed, would be more satisfactory if relating to animals 

 — several circumstances suggest that sex is a segregation- 

 phenomenon. Prof. Castle in a valuable essay has directed 

 attention to distinct evidence of disturbance in the heredity 

 of certain moths (Aglia tan and lugens, Standfuss's experi- 

 ments ; Tephrosia, experiments of Bacot and others, sum- 

 marised by Tutt),' where the disturbance is pretty certainly 

 connected with sexual differentiation. Mr. Punnett and I 

 are finding suggestions of the same thing in certain poultry 

 cases. Mr. Doncaster has pointed out that the evidence of 

 Mr. Raynor clearly indicates that a certain variety of 

 Abraxas grossiilariata, usually peculiar to the female, is a 

 Mendelian recessive. It is scarcely doubtful that this will 

 be shown to hold also for some other female varieties, e.g., 

 Colias ediisa, var. helice, &c. We can therefore feel no 

 doubt that there is some entanglement between sex and 

 gametically segregable characters. A curious instance of 

 a comparable nature is that of the Cinnamon canary 

 (\orduijn, &c.), and similar complications are alleged as 

 regards the descent of colour-blindness and haemophilia. 



In one remarkable group of facts we come very near to 

 the phenomenon of sex. Experiments made in conjunction 

 with Mr. R. P. Gregory have shown that the familiar 

 heterostylism of Primula is a phenomenon of Mendelian 

 segregation. Short style, or " thrum," is a dominant — 



1 Trans. Ent. .'ioc. Lond., 1S9S. 



NO. 18 I 7, VOL. 70] 



with a complication;' long style, or "pin," is recessive; 

 while equal, or "homostyle," is recessive to both. 



Even nearer we come in a certain sweet-pea example, 

 where abortion of anthers behaves as an ordinary Mendelian 

 recessive character." By a slight exaggeration we might 

 even speak of a hermaphrodite with barren anthers as a 

 " female." 



Consider also how like the two kinds of differentiation 

 are. The occasional mosaicism in Lepidoptera, called 

 " gynandromorphism," may be exactly paralleled by speci- 

 mens where the two halves are two colour-varieties, instead 

 of the two sexes. Patches of Silene inflata in this neigh- 

 bourhood commonly consist of hairy and glabrous in- 

 dividuals,^ a phenomenon proved in Lychnis to be depen- 

 dent on Mendelian segregation. The same patch consists 

 also of female plants and hermaphrodite plants. Is it not 

 likely that both phenomena are similar in nature? How 

 otherwise would the differentiation be maintained? The 

 sweet-pea case I have spoken of is scarcely distinguishable 

 from this. I therefore look forward with confidence to the 

 elucidation of the real nature of sex — that redoubtable 

 mystery. 



We now move among the facts with an altogether different 

 bearing. " Animals and Plants under Domestication," 

 from being largely a narration of inscrutable prodigies, 

 begins to take shape as a body of coherent evidence. Of the 

 old difficulties many disappear finally. Others are inverted. 

 Darwin says he would have expected " from the law of 

 reversion " that nectarines being the newer form would 

 more often produce peaches than peaches nectarines, which 

 is the commoner occurrence. Now, on the contrary, the 

 unique instance of the Carclew nectarine tree bearing 

 peaches is more astonishing than all the other evidence 

 together ! 



Though the progress which Mendelian facts make possible 

 is so great, it must never be forgotten that as regards new 

 characters involving the addition of some new factor to the 

 pre-existing stock we are almost where we were. When 

 they have been added by mutation, we can now study their 

 transmission ; but we know not whence or why they come. 

 Nor have we any definite light on the problem of adaptation ; 

 though here there is at least no increase of difficulties. 



Besides these outstanding problems, there remain many 

 special points of difficulty which on this occasion I cannot 

 treat — curiosities of segregation, obscure aberrations of 

 fertilisation ■" (occasionally met with), coupling of characters, 

 and the very serious possibility of disturbance through 

 gametic selection. Let us employ the space that remains 

 in returning to the problem of variation, already spoken of 

 above, and considering how it looks in the light of the new 

 facts as to heredity. The problem of heredity is the problem 

 of the manner of distribution of characters among germ- 

 cells. So soon as this problem is truly formulated, the nature 

 of variation at once appears. For the first time in the 

 history of evolutionary thought, Mendel's discovery enables 

 us to form some picture of the process which results in 

 genetic variation. It is simply the segregation of a new 

 kind of gamete, bearing one or more characters distinct 

 from those of the type. We can answer one of the oldest 

 questions in philosophy. In terms of the ancient riddle, we 

 may reply that the Owl's egg existed before the Owl; and 

 if we hesitate about the Owl, we may be sure about the 

 Bantam. The parent zygote, the offspring of which display 

 variation, is giving off new gametes, and in its gameto- 

 genesis a segregation of their new character, more or less 



1 It is doubtful if "thrum" ever breeds true, as both the other types 

 can do. Perhaps "thrum" is a //a«,ai« of De Vries. 



- Neglecting minor complications, the descent is as follows ; — Lady 

 Penzance ? x Emily Henderson (long pollen) £ gave purple Fi. In one 

 F2 family, with rare exceptions, coloured plants with darh axils were fer- 

 tife, those with light axils having i sterile, whites being eit her fertile or 

 sterile. The ratios indicated are 9 coloured, dk. ax., fertile S : i; coloured. 

 It. ax., sterile S : 3 white, fertile S : i white, sterile d. The fertile 

 whites, therefore, though llight-axilled (as whites almost always are), 

 presumably bear the dark-axil character, which generally cannot appear 

 except in association with coloured flowers. This can be proved next year. 

 Some at least of the plants with sterile S are fertile on the 9 side, and 

 when crossed with a coloured light-axilled type will presumably give only 

 light-axilled plants. 



"3 This excellent illustration was shown me by Mr. .K. W. Hill and 

 Mr. A. Wallis. A third form, glabrous, with hairy edges to the leaves. 



J In view of Ostenfeld's di 

 possibility that this phf 

 needs careful examinat 



i-ery of parthenogei 

 plays a part in son 



