SHORTER ARTICLES AND DISCUSSION 



DOES EVOLUTION OCCUR EXCLUSIVELY BY LOSS OP 

 GENETIC FACTORS ? 

 In an extremely interesting article, Professor Duerden^ has re- 

 cently discussed certain aspects of evolution in the light of obser- 

 vations on ostrich farming. He shows that as regards most char- 

 acters the germ plasm of the ostrich is remarkably stable and yet 

 that quantitative variation as regards wing and toe characters is 

 occurring and is being utilized, in particular for a gradual ameli- 

 oration of the valued plume characters. He believes that the 

 quantitative variation in question has a factorial genetic basis, a 

 view which I see no reason to question. He holds that repeated 

 selection m'ay probably extend the existing range of variation 

 downward, but not upward. In this last conclusion I can not 

 concur. It rests, I believe, on too close adherence to the "pres- 

 ence-absence hypothesis." It assumes that minus variation 

 occurs only by loss of factors and further that factors once lost 

 can not be recovered. I do not think that either of these assump- 

 tions will bear critical examination. Morgan has recorded, in 

 Drosophila, the occurrence of a reversed mutation by which col- 

 ored eyes were recovered in a white-eyed race, and on this ground 

 has questioned the validity of the entire presence-absence hy- 

 pothesis. I have found that in the piebald patterns of rats and 

 rabbits steady progress may be made by rei)cated selection in 

 changing the racial avcraizo fitlu r in ;) plus or u\ a niiiuis direc- 

 tion. Genetic chaniivs ;iiiVcriiiL:' ilh' cxTciii df tiu' iii'^rnt-iited 



as they are in the ease of iiiiinlx'i- of I'i'niiiiVN in tlh' ostficli wing, 

 but there is no indication that tlir cliaimvs ai'c t>\,'ius;vrly in a 

 minus direction, as Duerden a<Mniics ilinu to he m tlir ostrich. 

 He has observed variation in tlic uiiniluT of plimirs on th.' o>rrich 

 wing ranging from 33 to 42. lie assiinu's iliai ilu' va rial ion can 

 probably be carried below 33 by selection, tliroiinli nnnulation of 

 loss variations by dropping out of factors. I)iir that variation in 

 the opposite direction is not to be expected iMM-au-e 4l! is the 

 present maximum and factors for a liitrhei' ninnher liavniii- once 

 been lost can not be re-cov. red. Of eoiirse. tlie tliin- lo do m order 

 to test the validity of tin- view i- mve n an experiniental trial, 

 1 Duerden, J. E., " Tho (ierm Phusm of tlie Ostrioh," Amek. Nat., 53, 

 p. 312. 



565 



