November 24, 1922] 



SCIENCE 



589 



in viewpoint in the study of evolution very 

 well, and I cannot do better than quote him 

 again : 



Discussion of evolution came to an end prima- 

 rily because it was obvious that no progress was 

 being made. Morphology' having been explored in 

 its minutest corners, we turned elsewhere. Varia- 

 tion and 'heredity the two components of the evo- 

 lutionary path, were next tried. The geneticist 

 is the successor of the morphologist. We became 

 geneticists in the conviction that there at least 

 must evolutionary wisdom be found. We got on 

 fast. So soon as a critical study of variation was 

 undertaken, evidence came in as to the way in 

 which varieties do aetually arise in descent. The 

 unacceptable doctrine of the secular transforma- 

 tion of masses by the accumulation of. impalpable 

 changes became not only unlikely but gratuitous. 

 An examination in the field of the interrelations 

 of pairs of well characterized but closely allied 

 ' ' species ' ' next proved, almost wherever such an 

 inquiry eould be instituted, that neither could 

 both have been gradually evolved by natural selec- 

 tion from a common intermediate progenitor, nor 

 either from the other by suei a process. Scarcely 

 ever where such paiirs co-exist in nature, or occupy 

 conterminous areas do we find an intermediate 

 normal population as the theory demands. The 

 ignorance of comnion facts bearing on this part 

 of the inquiry which prevailed among evolution- 

 ists, was, as one looked back, astonishing and in- 

 esplicaible. It had been decreed that when vari- 

 eties of a species eo-exist in nature, they must be 

 connected by all intergradations, and it was an 

 article of faith of almost equal validity that the 

 intermediate form must be statistically the ma- 

 jority, and the extremes comparatively rare. The 

 plant breeder might declare that he had varieties 

 of Primula or some other plant, lately constituted, 

 uniform in every varietal character breeding 

 strictly true in those respects, or the entomologist 

 might state that a polymorphic species of a beetle 

 or of a moth fell obviously into definite types, 

 but the evolutionary pliilosopher knew better. To 

 him such statements merely showed that the re- 

 porter was a bad observer, and not improbably a 

 destroyer of inconvenient material. Systematists 

 had sound information but no one consulted them 

 on such matters or eared to hear what they might 

 have to say. X^e evolutionist of the eighties was 

 perfectly certain that species were a figment of 

 the systematist 's mind, not worthy of enlightened 

 attention. 



Then came the Meudelian clue. We saw the 

 varieties arising. Segregation maintained their 



identity. The discontinuity of variation was rec- 

 ognized in abundance. Plenty of the Mendelian 

 combinations would in nature pass the scrutiny 

 of even an exacting systematist and be given 

 "specific rank." In the light of such facts the 

 origin of species was no doubt a similar pheno- 

 menon. 



Now while it is true that genetics has by no 

 means solved the problem of evolution as yet, 

 and probably by itself never can and never 

 should have hoped to, the intensive pursuit of 

 this line of inquiry during the last decade has 

 enormously advanced our knowledge of general 

 biology. In the first place, thanks to the bril- 

 liant work of Morgan and his students with 

 Brosophila, we have firmly welded the last links 

 in the chain of a definite proof of ithe causal 

 connection between particular visible details of 

 nuclear structure in the germ cells and particu- 

 lar somatic characters transmitted from parent 

 to offspring in inheritance. The "mechanism 

 of heredity" is no longer a thing to speculate 

 and build broad nebulous hypotheses about. 

 We definitely know a good deal about this 

 mechanism and how it works. 



In the second place genetics, with cytology 

 as a working partner, as we have already noted, 

 has solved at least in broad outline, the prob- 

 lem of the causation of sex. In the third place, 

 the general results of modern genetic study 

 taken as a whole, and particularly the intensive 

 study of the breeding of animals and plants 

 which the getting of these results has entailed, 

 have made it highly probable, as I think most 

 geneticists, at least, will agree, that natural 

 selection as postulated by Darwin, has had but 

 little if anything directly to do with the causa- 

 tion of the evolution of the living things about 

 us. That natural selection is a process always 

 and everywhere going on in nature (except 

 in the case of eivilized man, where its operation 

 has been in large degree suspended by virtue 

 of certain attributes of civilization itself) no 

 competent observer of nature can possibly 

 deny. But that it either does or could bring 

 about evolutionary results attributed to it by 

 Darwin seems in tilie light of our present knowl- 

 edge, indefinitely more improbable than it did 

 twenty-five years ago. To give all the reasons 

 wliich exist to support this view would be 



