H 



NATURE 



[November 15, 19 17 



that a heritable variation ma;^ occur so small as to be 

 barely detectible. Although the variations do not 

 usually occur in this way, the case presents the condi- 

 tions which would allow of a gradual transition from 

 one extreme to the other, by means of numerous inter- 

 mediate conditions. In a population in which were 

 occurring such minute changes as are here shown to 

 be possible, we could get by selection such a continu- 

 ous series of gradations as Castle describes in his 

 rats. 



(2) But, as we have seen, the mutationists reject the 

 view that the changes in the coat colour of the rat are 

 due to alterations in a singe unit factor; they explain 

 this and other cases of the effectiveness of selection 

 on a single character by multiple modifying factors. 

 Accepting again their contention, the question is 

 shifted to the nature of such factors. 



Our direct experimental knowledge of these "modi- 

 fying factors" is scanty. We find data as to certain 

 known modifying factors by one of the workers on 

 Drosophila, Bridges (1916), in his recent important 

 paper on non-disjunction of the chromosomes.' Bridges 

 found a factor the only effect of which was to lighten 

 the eosin colour in a fly with eosin eyes; this factor, 

 indeed, nearly, or quite, turns the eosin eye white. 

 Another factor has the effect of lightening the eosin 

 colour a little less, giving a sort of cream colour. A 

 third factor dilutes the eosin colour not so much. In 

 addition to these, Bridges has discovered three other 

 diluters of the eosin colour, and another factor the 

 only effect of which is to modify eosin in the direction 

 of a darker colour. None of these factors has any 

 effect save on eosin-eyed flies. These things add tre- 

 mendously to our gradations in eye colour. We had 

 already been furnished seven grades, from white to 

 red; now we have seven secondary grades within a 

 single one of these seven primary grades. These 

 seven new grades are not located in the same unit 

 factor as are the seven primary ones ; their loci are 

 in other chromosomes (or possibly in other parts of 

 the same chromosome). 



Here again, then, we have minutely differing condi- 

 tions of a single shade of colour, brought about by 

 seven modifying factors. Bridges makes the follow- 

 ing remark concerning them : — 



"A remarkably close imitation of such a multiple 

 case as that of Castle's hooded rats could be con- 

 cocted with the chief gene eosin for reduced colour, 

 and these six diluters which by themselves produce no 

 effect, but which carry the colour of eosin through 

 every dilution stage from the dark yellowish pink of 

 the eosin female to a pure white." * 



Now this is an extremely interesting statement, one 

 that must arouse the keen interest of the student of 

 the method of evolution. In Drosophila we could get 

 the same sort of graded results that Castle does with 

 his rats, only in Drosophila this is by means of mul- 

 tiple modifying factors, whereas Castle believes that 

 in the rat it is by actual alterations of the hereditary 

 constitution ! 



But what are these modifying factors? And here 

 we come to the astonishing point. These modifying 

 factors are themselves alterations in the hereditary 

 constitution. Bridges leaves no doubt upon this point. 

 He lists and describes them specifically as mutations ; 

 as actual changes in the hereditary material. 



Where, then, is the difference in principle between 

 the condition in Drosophila and thai: in the rat? In 

 Drosophila there occur minute changes in the ger- 

 minal material, such as to give, so far as our present 

 imperfect knowledge goes, seven diverse grades of a 

 colour which is itself only one grade of another series 

 of seven known grades. By means of these graded 



7 Brdges, 1916, p. 148. (See Bibliography.) 



8 /did., p. 149. (See Bibliography.) 



NO. 2507, VOL. 100] 



changes one could obtain, by the mutationist's own 

 statement, the continuously graded results which selec- 

 tion actually gives. What more can the selectionist 

 ask? 



The mutationist thinks of all these numerous grades 

 as, after all, essentially discontinuous, as a series of 

 steps so minute that the difference between one and 

 the next one is not detectible. His opponent, on the 

 other hand, perhaps thinks of the series as actually 

 continuous. But when steps become so minute as to 

 be beyond detection, the question whether they exist 

 becomes metaphysical. 



To put the case in brief, if the mutationists are to 

 show that the existence of multiple modifying factors 

 has any bearing on the general question of the effec- 

 tiveness of selection, they must show that such factors 

 are not themselves minute changes in the hereditary 

 constitution. Not only have they made no attempt to 

 do this, but in the only well-examined cases they state 

 squarely that such factors are indeed alterations in 

 the hereditary constitution. 



For the inheritance of such factors as Mendelian 

 units, of course absolutely nothing is required save 

 that the location of the change is in a chromosome. 

 No particular degree of magnitude, no unity of any 

 other kind is required. 



But there remains one point brought out by the 

 mutationists which is of great importance to the 

 student of the method of evolution. While they must 

 admit, by their own account, that all these grades 

 occur, they, of course, point out that the changes do 

 not occur in a continuous series. In the eye of Droso- 

 phila variation may occur from red to white directly, 

 without any transitional stages ; or from any grade 

 to any other; the continuous scale is obtained only 

 by arranging the steps in order. Therefore, it is 

 maintained, evolution may have occurred by such 

 large steps, not by continuous gradations.* This is, 

 of course, a matter deserving of serious consideration. 

 But certain other points must be considered also. 

 First, the very facts known for Drosophila show that 

 there is nothing to prevent a passage from one extreme 

 to the other by minute changes, just as is held to 

 occur by the palaeontologists and selectionists, although 

 change by large steps occurs also. Secondly, in such 

 cases as the eye colour of Drosophila we are dealing 

 with characters that are already highly developed. We 

 know, for example, that this particular character is 

 formed by the co-operation of many separate parts of 

 diverse chromosomes ; it is a highly complex product 

 of evolution. Now, we find that one or another of 

 these parts may suddenly cease to perform its function, 

 so that the red colour is not completely formed ; there 

 is a sudden change in it; or it may disappear entirely. 

 But is this, after all, strong evidence that in the 

 original production of this complex character with its 

 numerous underlying functional parts, there was the 

 same change by sudden large steps? Indeed, is it not 

 rather true that such destructive changes in a fully 

 formed character could not be expected to throw light 

 on how that character was built up? 



To sum up, it appears to me that the work on 

 Drosophila is supplying a complete foundation for 

 evolution through selection of minute gradations. The 

 so-called "multiple allelomorphs" show that a single 

 unit factor mav thus exist in a great number of 

 grades ; the "multiple modifying factors " show that 

 a visible character may be modified in the finest grada- 

 tions by alterations in diverse parts of the germinal 

 apparatus. The objections raised by the mutationists 

 to gradual change through selection are breaking;!: down 

 as a result of the thoroughness of the mutat'onists' 

 own studies. The only outstanding diflficulty is the 



S See particularly the discussion of this point in Morgan, 1916, pp. 7-27. 

 (See Bibliography.) 



