196 



NATURE 



[November 8, 19 17 



ancestry. He supports his colleague, Dr. W. D. 

 Matthew — in opposition to the view generally held in 

 this country — in regarding the lower jaw of Eoan- 

 thropus as that of a Piltdown chimpanzee associated 

 by a curious chance with the Piltdown man in a pocket 

 of gravel. We look forward to the appearance of 

 parts iii. and iv. of Dr. Gregory's studies, in which 

 he proposes to review the phylogenies of the catar- 

 rhine, or Old World, monkeys, and platyrrhine, or 

 New World, monkeys and Lemuroids. 



HEREDITARY CHARACTERS IN RELATION 

 ■ TO EVOLUTION. 



PROF H. S. JENNINGS, of the Johns Hopkins 

 University, delivered a lecture on March 15 be- 

 fore the Washington Academy of Sciences on " Ob- 

 served Changes in Hereditary Characters in Relation 

 to Evolution." This lecture, published in the Journal 

 of that Academy (vol. vii., No. 10), consists of a 

 discussion on the factors of evolution of such great 

 interest that we have decided to print an abridgment 

 so that readers of Nature may have the opportunity 

 of studying and appreciating his arguments as set 

 forth in his own words. The older school of biologists 

 in this country will doubtless welcome Prof. Jennings's 

 brilliant and ingenious interpretation of the recent 

 work of American zoologists on genetics, so as to 

 support the Darwinian interpretation of the evolu- 

 tionary process. Prof. Jennings's criticism of Mr. 

 Bateson's British Association address (19 14) leaves the 

 reader in doubt whether he has appreciated the view 

 that the " loss, and disintegration " in the germ-plasm 

 are conceived by Bateson as the shedding of successive 

 inhibitory factors the withdrawal of which leaves the 

 hypothetical fundamental germ-complex free to pro- 

 duce an increasingly complex result in the developing 

 organism. 



The problem of the method of evolution is one 

 which the biologist finds it impossible to leave alone. 

 Can we bring the facts which experimental work has 

 brought out into relation with the method of evolu- 

 tion? 



What we may call the first phase of the modern 

 experimental study of variation is that which cul- 

 minated in the establishment of the fact that most 

 of the heritable differences observed between closely 

 related organisms — ^between the members of a given 

 species, for example^are not variations in the sense 

 of alterations ; are not active changes in constitution, 

 but are permanent diversities ; they are static, not 

 dynamic. This discovery was made long ago by the 

 Frenchman Jordan ; but, as in the case of Mendelism, 

 science ignored it and pursued cheerfully its false path 

 until the facts were rediscovered in recent years. All 

 thorough work has led directly to this result : that 

 any species or kind of organisni is made up of a 

 very great number of diverse stocks, differing from 

 each other in minute particulars, but the diversities 

 inherited from generation to generation. This result 

 has in recent years dominated all work on the occur- 

 rence of variations ; on the effects of selection ; on the 

 method of evolution. The condition is particularly 

 striking in organisms reproducing from a single 

 parent, so that there is no mixing of stocks ; I found 

 it in a high degree in organisms of this sort which 

 I studied. Thus the infusorian Paramecium I found 

 to consist of a large number of such heritably diverse 

 stocks, each stock showing within itself many varia- 

 tions that are not heritable.^ Difflugia corona shows 

 the same condition in a marked degree.^ A host of 

 workers have found similar conditions in all sorts of 



1 Jennings, 1908-11. (See Bibliography.) 



2 Jennings, 1916. (See Bibliography.) 



NO. 2506, VOL. 100] 



organisms. It led to the idea of the genotype 

 (Johannsen), as the permanent germinal constitution 

 of any given individual; it supported powerfully the 

 conception of Mendelism as merely the working out of 

 recombinations of mosaic-like parts of these permanent 

 genotypes. The whole conception Is in its essential 

 nature static; alteration does not lit into the scheme. 



This discovery seemed to explain 'fully all the 

 observed effects of selection within a. species ; but gav« 

 them a significance quite the reverse of what they 

 had been supposed to have. It seemed to account for 

 practically all the supposed variations that had been 

 observed; they were not variations at all, in the sense 

 of steps in evolution ; they were mere instances of the 

 static condition of diversity that everywhere prevails. 

 Jordan, the devout original discoverer of this condi- 

 tion of affairs, maintained that it showed that organ- 

 isms do not really vary ; that there is no such process 

 as evolution ; and, Indeed, this seems to be the direct 

 logical conclusion to be drawn. 



Now, this multiplicity of diverse stocks really repre- 

 sents the actual condition of affairs, 50 far as it goes. 

 Persons who are interested in maintaining that evolu- 

 tion is occurring, that selection is effective, and the 

 like, make a very great mistake in denying the exist- 

 ence of the condition . of diversity portrayed by the 

 genotypists. What they must do is to accept that 

 condition as a foundation, then show that it Is not 

 final ; that It does not proceed to the end ; that the 

 diverse existing stocks, while heritably different as the 

 genotypists maintain, may also change and differen- 

 tiate, in ways not yet detected by their discoverers. 



But, of course, most of the adherents of the "ortho- 

 dox genotype theory" do not maintain, with their first 

 representative Jordan, that no changes occur. Typi- 

 cally, they admit that mutations occur; that the geno- 

 type may at rare intervals transform, as a given 

 chemical compound may transform into another and 

 diverse compound. We all know the typical Instances : 

 the transforming mutations of CEnothera : the bud 

 variations that show In a sudden change of colour or 

 form In plants ; the dropping out of definite Mendelian 

 units in Drosophila and elsewhere; the transformation 

 of particular Mendelian units Into some other condi- 

 tion. 



So much, then, may serve as an outline of a pre- 

 vailing theory ; organisms forming a multitude of 

 diverse strains with diverse genotypes; the genotype 

 a mosaic of parts that are recombined in Mendelian 

 Inheritance; selection a mere process of Isolating and 

 reccwnbining what already exists ; large changes occur- 

 ring at rare intervals, through the dropping of bits 

 of the mosaic, or through their complete chemical 

 transformation ; evolution by saltations. 



Certain serious difficulties appear in this view of the 

 matter ; I shall mention merely two of them, for their 

 practical results. One is the very existence of the 

 minutely differing strains, which forms one of the 

 main foundations for the genotype theory. How have 

 these arisen? Not by large steps, not by saltations, 

 for the differences between the strains go down to the 

 very limits of detectlbility. On the saltation theory, 

 Jordan's view that these things were created separate 

 at the beginning seems the only solution. 



Secondly, to many minds there appears to be an 

 equally great difficulty in the origin by saltation of 

 complex adaptive structures, such as the eye. I shall 

 not analyse this difficulty, but merely point to jt and 

 to the first one mentioned, as having had the prac- 

 tical effect of keeping many Investigators persistently 

 at work looking for something besides saltations as a 

 basis for evolution ; looking for hereditary changes 

 that would permit a continuity in transformation. 



Where reproduction is from a single parent we 

 meet the problem of Inheritance and variation in its 



