February 9, 1922] 



NATURE 



^IS 



a difference which has arisen at some time through 

 a variation? We can determine characters only by 

 l^omparing related organisms and noting their differ- 

 ices. To say that all characters are alike, then, is 

 say that all variations are alike, which Sir Archdall 

 laid himself admits is not the case. 



Perhaps an experimental instance will make this 

 learer. Some years ago a fasciated specimen of an 

 inothera was sent to me. The plant was in seed ; 

 le stem was about 2 in. wide at the widest part 

 id as flat as a ribbon. It was, of course, impossible 

 say with certainty, from inspection, whether this 

 laracter would be inherited or not, although the 

 robabilities were somewhat against it. I sowed the 

 jds, large numbers of them, and they all gave rise 

 perfectly normal plants with round stems. The 

 laracter was therefore non-inherited in this particular 

 ise. It is, of course, well known that fasciations 

 lay be produced by excessive nutrition, and that the 

 Bculiarity is then, as a rule at least, not inherited. 

 But there are other instances in which this character 

 inherited. For example, in the common coxcomb 

 gardens, Celosia cristata, fasciation is one of the 

 jcific characters, distinguishing it from such species 

 Celosia plumosa, in which the stems do not fasciate 

 ider ordinary conditions of cultivation. I have often 

 rown these two species in quantity side by side in 

 i»e greenhouse, and compared the extreme fasciation 

 C. cristata with the ordinary branched character 

 of the other species. It should be mentioned, how- 

 ever, that C. plumosa does sometimes show slight 

 fasciation at the tips of the branches, and this can be 

 « xaggerated by growing the plants under conditions 

 iif very high temperature and moisture. But it never 

 pproaches the degree of fasciation found constantly 

 s a specific (and therefore inherited) character in 

 ( '. cristata. 



The same character, fasciation, is therefore clearly 

 inherited in C. cristata, but it was not inherited in 

 the particular instance in Oenothera which I tested. 

 It is also clear that the fasciated Celosia must have 

 originated at some time as a variation from plants 

 with: normal stems. Innumerable similar instances 

 will be known to experimental biologists, and it is 

 such cases which they have in mind when they speak 

 of characters as of two kinds, inherited and non- 

 inherited. When a particular new character appears 

 as the result of a variation no one can predict with 

 certainty whether it will be inherited or not until the 

 organism which shows it is tested. But, of course, 

 probabilities may be stated by comparison with similar 

 characters the hereditary behaviour of which is already 

 known. In the face of such experimental facts, 

 which are well known to all geneticists, it is futile to 

 state that all characters are equally acquired and 

 equally inherited. 



When Sir Archdall Reid implies that combs and 

 corns are equally inherited he forgets a whole class 

 of experimental facts such as those above cited. One 

 must refuse to consider corns as inherited, because 

 there always remains the possibility that a case may 

 arise where, through a germinal change, they are 

 inherited without any special stimulus to produce 

 them. The inherited condition known as keratosis 

 i>, indeed, an epidermal thickening of similar 

 character. It seems clear that moles are not usually 

 inherited, but if the writer in Nature is correct (see 

 Nature, January 19, p. 78), then there may be 

 1 stances in which even a mole is inherited in the 

 iiitimate sense in which the term " inheritance " is 

 customarily used by biologists. 



R. RuGGLES Gates. 

 King's College, University of London, 

 January 27. 

 NO. 2728, VOL. 109] 



Sir Archdall Reid's letter in Nature of January 26 

 will render considerable service if it induces students 

 of evolutionary phenomena accurately and precisely 

 to define their terms. 



If one may, at the beginning, set forth two general 

 statements, the ground will be cleared for a discus- 

 sion of Sir Archdall Reid's points : — 



(i) Genes or factors are inherited, characters are 

 I not. 



I (2) A gene conditions the appearance in the organism 

 , of a character or group of characters. 



(3) The effect produced by a gene in the organism 

 depends on the environmental conditions which prevail 

 during the life-history of the organism and on the 

 other genes which the organism possesses. 



To show that characters are not inherited, the 

 example of "abnormal abdomen " in Drosophila may 

 be cited. 



The gene for " abnormal abdomen " causes the 

 condition in moist cultures only. In dry cultures the 

 flies hatch out normal in appearance. 



The statement that rose comb and single comb are 

 not more inheritable than corns on oarsmen's hands 

 is obviously correct. Any capacity for reacting to a 

 stimulus may be considered as being represented in 

 the chromosomes by a gene or genes. In this case 

 we may assume that the capacity for responding to 

 the frictional stimulus of the oar by forming a mass 

 of proliferated tissue on the palms of the hands is 

 inherited. 



Certain other points raised by Sir Archdall Reid 

 may be dealt with briefly : 



(i) The impure dominant does not inherit any trait. 

 It inherits the recessive gene from one parent which 

 may or may not interact with environment and with 

 other genes to produce an effect. The terms 

 "dominant" and "recessive" are purely arbitrary, 

 and used only for convenience. 



(2) The pure extracted recessive inherits a recessive 

 gene from one parent and a similar recessive gene 

 from the other. The germ-cells of an impure 

 dominant carry either the dominant or the recessive 

 gene. 



(3) The ancestral condition obtained in some pigeon 

 crosses is due to the interaction of the two sets of 

 genes contributed by the two parents. 



The interaction of genes may be illustrated by an 

 example from the cow-pea. A red cow-pea crossed 

 with a _ white may give a black in the first hybrid 

 generation. White possesses a gene for black which is 

 without effect except in the presence of the gene for 

 red present in the red parent. At least eight different 

 genes in the cow-pea are known to depend for their 

 expression on a single colour-conditioning gene. 



S. C, Harland. 



[Sir Archdall Reid began this correspondence with 

 a letter in Nature of November 25, 1920; and we 

 have now invited him to close it. — Editor.] 



The Radiant Spectrum. 



Dr. Hartridge's objections to my explanation of 

 this phenomenon (Nature, September i, p. 12, and 

 December 8, 192 1, p. 467) seem to be based on an im- 

 perfect appreciation of Brewster's observations on 

 the subject. Brewster brings out two facts clearly 

 in his paper : First, when a very small and intense 

 source of white light is viewed directly by the eye 

 it appears surrounded by a system of radiating 

 streamers which appear to diverge directly from it; 

 secondly, when a prism of small dispersive power is 

 interposed in front of the eye the streamers are 

 deviated and now apf>ear to divcge from a point 

 lying beyond the violet end of the spectrum into 



