November 8, 1917] 



NATURE 



197 



simplest form ; for there is nothing which complicates 

 genetic problems so enormously as does the continual 

 mixing of diverse stocks in biparental inheritance. In 

 uniparental reproduction we have but one genotype to 

 deal with ; we can be certain that no hereditary char- 

 acters are introduced from outside that genotype. 



To hope for results on the problem in which we 

 are interested, we must resolve to carry on a sort 

 of second degree research, as it were. We must take 

 a single stock — choosing an organism that is most 

 favourable- for such work — then proceed to a most 

 extensive and intensive study of heredity, of variation, 

 and of the effects of selection for long periods within 

 such a stock. 



Such an organism, most favourable from all points 

 of view, I found in the rhizopod Difflugia corona. 

 It has numerous distinctive characters, all congenital, 

 all inherited in a high degree, yet varying from parent 

 to offspring also ; none of these characters changed by 

 growth or environmental action during the life of the 

 individual. 



Long-continued work showed that a single strain of 

 this animal, all derived by fission from a single parent, 

 does differentiate gradually, with the passage cf 

 generations, into many hereditarily diverse strains. 

 The important facts about the hereditary variations 

 and their appearance are the following : — 



(i) Hereditary variations arose in some few cases by 

 rather large steps or " saltations." 



(2) But the immense majority of the hereditary 

 variations were minute gradations. Variation is as 

 continuous as can be deLected. 



(3) Hereditary variation occurred in many different 

 ways, in many diverse characters. There was no 

 single line of variation followed exclusively, or in the 

 overwhelming majority of cases. 



(4) It gave rise to many diverse combinations of 

 characters : large animals with long spines ; small 

 animals with long spines; large animals with short 

 spines ; small animals with short spines ; and so on. 

 Any set of characters might vary independently of the 

 rest. 



(5) The hereditary variations which arose were of 

 just such a nature as to produce from a single strain 

 the hereditarily different strains that are found in 

 nature.' 



I judge that if the intermediate strains were killed, 

 the two most diverse strains found in Nature might 

 well be classed as different species, although the ques- 

 tion of what a species is must be left to the judgment 

 or fancy of the individual. 



How do these results compare with those found by 

 other men? If we take a general survey, we find the 

 following main classes of cases : — 



(i) First, we have the mutations of CEnothera and 

 its relatives : large transformations occurring sud- 

 denly. 



(2) Secondly, we have a large miscellaneous collec- 

 tion of mutations observed in various classes of organ- 

 isms : "bud variations," dropping out of unit factors, 

 and the like — all definite saltations, but not genetically 

 fully analysed. 



(3) In Drosophila as studied by Morgan and his 

 associates, we have the largest and most fully analysed 

 body of facts which we possess with respect to changes 

 in hereditary character in any organism. The changes 

 here are pictured as typical saltations ; but of these 

 I shall speak further. 



(4) In palaeontology, as the results are presented in 

 recent papers by Osborn,* the evidence is for evolution 

 by minute, continuous variations which follow a single 

 definite trend. 



(5) Finally, we have the work in biparental inherlt- 



^ The full account of this work is given in Jennings, 1916. (See Biblio- 

 graphy.) 

 ■• Osbom, 1912, 1915, 1916. (See Bibliography.) 



NO. 2506, VOL, 100] 



artce from Castle and his associates * ; this gives evi- 

 dence for continuous variation, not following a single 

 necessary trend, but guided by external selection. 



Furthermore, we discover in our survey that there 

 are at least two well-marked controversies in flame at 

 the present time : — 



First, we have the general controversy between, on 

 one hand, those who are mutationists and adherents 

 of the strict genotype view ; on the other, those who, 

 like Castle, believe that we observe continuous heredi- 

 tary variations in the progress of biparental reproduc- 

 tion. The mutationists attempt to show that the 

 apparent gradual modification of characters observed 

 in breeding is in reality a mere working out of Men- 

 delian recombinations. 



Secondly, we have a somewhat less lively controversy 

 between the genotypic mutationists and the palaeonto- 

 logical upholders of evolution toy continuous variation. 



Now let us look briefly into the points at issue in the 

 controversy between the "'genotypic mutationists" and 

 the upholders of gradual change during biparental in- 

 heritance. 



Castle finds that in rats he can, by selection, gradu- 

 ally increase or decrease the amount of colour in the 

 coat, passing by continuous stages from one extreme 

 to the other. As to this, he holds two main points : — 



(i) The change is an actual change in the hereditary 

 characteristic of the stock; not a mere result of tbe 

 recombination of Mendelian factors. This is the 

 general and fundamental point at issue. 



(2) More specifically, he holds it to be an actual 

 change in a single-unit factor; this single factor 

 changes its grade in a continuous and quantitative 

 manner. 



On the other side, the critics of these views maintain 

 that the changes shown are not actual alterations in 

 the hereditary constitution at all, but are mere results 

 of the recombinations of Mendelian factors. And 

 specifically, they find a complete explanation of such 

 results as those of Castle in the hypothesis of multiple 

 modifying factors. There is conceived to be a single 

 "main factor" which determines whether the "hooded 

 pattern" shall, or shall not, be present. In addition 

 to this there are a considerable number of "modify- 

 , ing factors " which, when the " hooded pattern " is 

 present, increase or decrease the extent of pigmenta- 

 tion. When many of the positive factors of this sort 

 are present, the rat's coat has much pigment; when 

 fewer are present the extent of pigment is less, and 

 so on. The process of changing the extent of pig- 

 mentation by selection cortsists, according to this view, 

 merely in making diverse combinations of these fac- 

 tors, by proper crosses. 



This same explanation is applied to a great variety 

 of cases. Castle had carried the war into the enemy's 

 country by predicting (or at least suggesting) that the 

 so-called unit characters in Drosophila would be found 

 to be modifiable through selection. ** Later research 

 by MacDowell (1915), Zeleny and Mattoon (1915), 

 Reeves (1916), Morgan (1917), and Sturtevant (1917) 

 actually verified this prediction ; it has indeed been 

 found that the Drosophila mutations can be modified 

 by selection. Again, the mutationists counter the 

 blow with their explanation of multiple modifying 

 factors, which are segregated in the process of selec- 

 tion ; and they give some real evidence that such is 

 actually the case. What I am going to do is to aban- 

 don the ground that Castle would defend, proceed 

 directly into the territory of the enemy, accept the 

 conditions met there, then see where we come out in 

 relation to the nature of variation, the effects of selec- 

 tion, and the method of evolution. 



In no other organism have heritable variations been 



■^ Ca«:tle, iQiSrt, 1916, igi6«, 19163, 1917; Castle and Phillips, 1914, etc 

 (See Bibliography.) 



'' Castle, 1915, p. 39. (See Bibliography.) 



