October io, 1912J 



NATURE 



179 



different genetical behaviours of these heterozygous 

 first generations give the clue to the difference between 

 the two forms of white used as parents. In the 

 former case — that in which the first (Fj) generation 

 consists of coloured offspring — the second (F„) genera- 

 tion, raised by self-fertilising Fj individuals or by 

 crossing them with one another, consists of coloured : 

 white in the proportion of 3 : i. 



Whence we conclude that the white used in this 

 experiment owes its character of whiteness to lack 

 of the pigment-producing factor which is present in 

 the coloured parent race. This conclusion is confirmed 

 by the genetical behaviour of the whites of the F^ 

 generation. Such extracted whites breed true to 

 flower-character — that is, give rise to white-fiowered 

 offspring only. White-flowered races which behave in 

 this manner are termed recessive whites. 



In the second case — that in which the Fj generation 

 consists of white-flowered offspring — the Fj genera- 

 tion, from selfed or intercrossed F\ plants, consists 

 of three white : one coloured. The coloured offspring 

 breed true. Of the three whites one breeds true to 

 whiteness and the other two give rise, like the white 

 Fj generation, to three white : one colour. White races 

 which thus impose their whiteness on the offspring 

 of their union with a coloured race are Icnown as 

 dominant whites. Mendelians account for the genetical 

 behaviour of dominant whites by assuming that they 

 carry the character for colour and also a character for 

 colour-inhibition. This hypothesis, which is novel to 

 biology, is amply justified by genetical results. It 

 propounds a series of questions to the physiologist 

 and biochemist, and in so doing exemplifies the fruit- 

 fulness of Mendelism. We shall see immediately 

 ■whether the biochemist is able to take up this Men- 

 delian challenge and what answer he can give to it. 



At present, however, we are concerned to show by 

 an example the necessity of prefacing the study of 

 variation bjf Mendelian analysis. It was stated just 

 now that the cross, dominant white by colour, results 

 in a white F,. That statement requires amplification. 

 Grown under normal conditions the F, individuals bear 

 pure white flowers ; but if grown in somewhat higher 

 temperatures the flowers develop a distinct tliough 

 pale flush of colour. It is easy to show that the 

 factor for colour is unaffected by the changed condi- 

 tions, for the flushed F. individuals yield offspring of 

 the same kind and in the same proportions as those 

 produced by white Fj plants. 



It is fairly evident that the flushing is produced by 

 the destructive action of heat on the inhibitor. In 

 pre-Mendelian times this response to temperature 

 would have been added without more ado as yet 

 another ornament to dress the window of that old 

 curiosity shop which is stocked w'ith miscellaneous and 

 heterogeneous articles all ticketed with the label 

 "variation." 



But in the light of Mendelism we may see in this 

 effect of temperature the result of the casting-vote of 

 circumstance on a heterozygous constitution. We may 

 recall instances — as, for example, those provided by 

 the well-known experiments on the effects of high 

 temperatures on insect larvae — which seem to show 

 that environmental agencies may single out not only 

 characters but also factors for attack. Thus we may 

 begin to cohere in series the hitherto sundered and 

 scattered phenomena of variation. 



It is not yet possible to say how much of variation 

 is to be put down to the interplay of characters, or, 

 rather, to the differential effects of external conditions 

 on characters which tend to balance one another ; 

 but this at least may be said — that the old and worn 

 controversy on acquired characters was so much waste 

 of words, because the problem purporting to be dis- 



No. 2241, VOL. go] 



cussed had never been defined. Like the half of 

 human quarrels, it was a quarrel about words. 



It is stated in the books that the formation of 

 peloric (regular) flowers may be induced by uniform 

 illumination. Was the material used in the research 

 homozygous or heterozygous? Does uniform illu- 

 mination just prevent the unpaired factor from induc- 

 ing normal growth? If so, what is the effect on the 

 homozygous normal ? These are examples of ques- 

 tions which suggest themselves at every turn, and 

 they will abide the answer of experiment. The time 

 is approaching when it will be possible to test the 

 validitv of the hypothesis on which the superhypothesis 

 of natural selection rests apparently secure from veri- 

 fication or disproof. 



That hypothesis maintains that everything is in a 

 state of flux ; that variation occurs at all times and 

 affects all parts. This may be true of multiple 

 mongrels ; of organisms which are heterozygous for 

 many characters. On the other hand, nothing is 

 more surprising than the stability of forms which are 

 pure-bred for a fair number of characters, and it is 

 at all events a suggestion not to be rejected sum- 

 marily that plants pure bred for a considerable number 

 of characters may exhibit a constancy and stability 

 not usually associated with our ideas of living things. 



In any case, it is open to the biologist to provide 

 himself with suitable material wherewith to study the 

 range and. scope of variation and to investigate the 

 conditions under which the organism discards old 

 characters and regresses or acquires new ones and 

 progresses. It is open to the Mendelian breeder to 

 standardise creation. 



Thus in fulfilling the first part of its task — that of 

 defining the pure-bred — the Mendelian method has 

 provided the material for the fulfilment of the second 

 part — namely, the investigation of the conditions which 

 make for the stability and instability of the organism. 

 I think the time has come when this latter task 

 might be undertaken on a large scale and with good 

 prospects of success. 



So far I have played the part of one of those street- 

 corner watchers of the skies who offer a telescope for 

 the inspection of the heavens. I have now to take 

 a turn myself, and by means of the binoculars of 

 Mendelism' and physiology survey, not the celestial 

 bodies, but certain new features of a small and narrow 

 terrestrial field which this instrument brings within our 

 ken. My survey has reference to the phenomena of 

 the pigrnentation of plants, and is confined to those 

 presented by the anthocyan or sap pigrrients to which 

 the colours of many flowers are due. 



Until recently knowledge of the processes of pig- 

 mentation advanced £llong two main and independent 

 lines. One line of advance — that follovi-ed with such 

 brilliant success by Bateson and the Cambridge school, 

 as well as by other students of genetics — has led to a 

 wealth of exact knowledge with respect to the factors 

 and characters which determine coloration. Tlie other 

 line of advance, pursued v.'ith no less brilliant results 

 by Chodat and Bach and by Palladin and his asso- 

 ciates, has resulted in a great increase of our under- 

 standing of the biochemistry of pigmentation. 



The merit of being the first to combine the genetical 

 with the biochemical method belongs to Miss Whel- 

 dale, to whom, moreover, we owe a good working 

 hypothesis of the nature of the processes involved in 

 pigment-formation. The work of Palladin and ot 

 Chodat and Bach is so well known that I need not 

 review it in any detail. To Palladin we owe in large 

 measure the conception that respiration consists in a 

 sequence of enzyme-like actions, the later of which 

 result in oxidations and are ascribed to oxydases. 

 To the same observer we owe also the suggestion 



