402 



NATURE 



\AiLgiist 25, 1887 



modifications are inherited, or that they are not] must have upon 

 our views of Life, Mind, Morals, and Politics, the question — Which 

 of them is true ? demands, beyond all other questions whatever, 

 the attention of scientific men." 



That functionally- produced modifications are inherited was the 

 great assumption upon which Lamarck founded his theory of 

 evolution. Erasmus Darwin adopted the assumption, and it was 

 also accepted by Charles Darwin as representing a highly im- 

 portant factor of organic evolution, although subsidiary to that of 

 natural selection. Lastly, Mr. Spencer has always upheld the 

 assumption, and, as we shall subsequently see, has done more 

 than anybody else in the way of its justification. On the other 

 hand, of late years a growing tendency has been displayed by 

 those evolutionists who out-Darwin Darwin, not only to assign 

 to natural selection a monarchical government over the whole 

 realm of organic Nature, but also, and consequently, to deprive 

 use and disuse of those lesser sovereignties which were so freely 

 accorded to them by the " Origin of Species." This tendency has 

 now reached a climax in the publication of an essay, by na less 

 an authority than Prof. Weismann, wherein the Lamarckian 

 principles of use and disuse are denied in toto.^ We may there- 

 fore best begin our stock-taking of the whole subject by consider- 

 ing what Prof. Weismann has said ; for assuredly the doctrine of 

 use and disuse as themselves useless could nowhere meet with an 

 abler champion. 



In the first place, he is committed to this doctrine as a 

 necessary consequence of his own theory of heredity, according 

 to which any change acquired hy the individual cannot be trans- 

 mitted to progeny. This theory regards the individual organism 

 as nothing more than what may be termed a temporary receptacle 

 of "germ-plasma" — this germ-plasma being handed on from 

 generation to generation, without ever being affected by any 

 changes that may take place in the organisms which contain it. 

 And the only reason why such appears to be the case — or why 

 in the course of generations one specific type gradually changes 

 through inherited modifications into another — is because the 

 germ-plasma itself is liable to variation, "and when the variations 

 happen to be of a kind which lead to favourable modifications of 

 the ^ tore-houses (organisms), these store-houses are preserved 

 by natural selection, and with them the peculiar variations of the 

 germ-plasma, which are thus carried on to the next generation. 

 Hence natural selection is really at work upon variations of the 

 germ-plasma, and hence also no change occurring in an organism 

 during its own life-history can at all aflfect its progeny — any 

 more, for instance, than the chipping or the twisting of a vessel 

 can modify the chemical constitution of whatever substance the 

 vessel may contain. In short, it is only so-called congenital 

 variations — or variations of germ-plasma- — that can be inherited ; 

 and, therefore, it is only upon such variations that survival of 

 the fittest is able to act. All variations afterwards superinduced 

 in the organism — whether by way of mutilation, disease, acquisi- 

 tion of faculty, or degeneration of structure — are destined to be 

 immediately extinguished by the death of the organism. Now, 

 from this general theory it necessarily follows that the effects of 

 use and disuse in the individual cannot be transmitted to 

 progeny ; for, if they could, the fact would be fatal to the 

 theory. Hence it is, as above observed, that Prof. Weismann is 

 committed by his theory of heredity to a denial of the Lamarckian 

 assumption, which, as we have seen, was accepted by Darwin. 



But besides this merely a priori ground of deduction from 

 his own theory. Prof. Weismann stands upon the affirma- 

 tion that there is, as a matter of fact, no real evidence 

 of the effects of use and disuse being inherited. For, he 

 maintains, all the supposed evidence on this head admits of 

 being fully interpreted by quite another principle. When an 

 organ (or any structure) falls into disuse, in the course of genera- 

 tions it atrophies, becomes rudimentary, and finally disappears. 

 This fact is generally taken as proof of the inherited effects of 

 disuse— -seeing that it is so strikingly analogous to these effects in 

 the case of individual organisms. But there is an alternative 

 possibility. The raison d'etre of the organ before it fell into 

 disuse, was its utility : it was originally built up under the 

 nursing influence of natural selection solely on account of its 

 serviceability. When therefore from changed conditions of life, 

 or for any other reason, the organ ceased to be serviceable, the 

 premium which had been previously set upon it by natural 

 selection was withdrawn ; individuals which happened to present 

 the organ qf a size below the average were no longer eliminated 

 in the struggle for existence, but were allowed to propagate. 

 Thus, by free intercrossing, the average size became less and 

 ' " Ueber den Riickschritt in der Natur" (Freiburg, 1886). 



less in every succeeding generation, until eventually, according 

 to Weismann, it must altogether disappear. In thort, as the 

 organ was originally built up by natural selection, when natural 

 selection was withdrawn, is any other explanation required of 

 the fact that the organ progressively d\\ indled ? 



Unknown to Prof. Weismann, this principle, under the name 

 "Cessation of Selection," was enunciated by the present writer 

 in a series cf articles published in these pages so long ago as 

 1873-74. Attention is now drawn to this fact merely for the 

 sake of informing biologibts that the principle met with the full 

 approval of the late Mr. Darwin, and also to state exactly the 

 shape in which it was thus approved by him. For in one or 

 two particulars the idea as published in Nature differs from 

 that w hich has been recently and independently arrived at by 

 Prof Weismann. As the issues of Nature in question are out 

 of print, and as the matter cannot be more briefly stated now 

 than it was stated then, I may best begin by reprinting the 

 portion of these articles which sets forth the principle of the 

 cessation of selection, as this was accepted by Mr. Darwin. 



"In a former communication (Nature, vol. ix. p. 361) I 

 promised to advance what seemed to me a probable cause — 

 additional to those ah eady known — of the reduction of useless 

 structures. As before stated, it was sugge?ted to me by the 

 penetrating theory proposed by Mr. Darwin (Nature, vol. viii. 

 pp. 432 and 505), to which, indeed, it is but a supplement.^ 

 Epitomising Mr. Darwin's conception as the dwarfing influence 

 of impoverished conditions progressively reducirg the average 

 size of a useless structure by means of free intercrossing, the 

 present cause may be defined as the mere cessation of the selec- 

 tive influence from changed condition of life. 



" Suppose a i-truciure to have been raised by natural selection 

 from o to average size 100, and then to have become wholly use- 

 less. The selective influence would now not only be withdrawn, 

 but reversed ; for, through Economy of Growth— understanding 

 by this term both the direct and the indirect influence of natural 

 selection — it would rigidly eliminate the variations loi, 102, 103, 

 &c., and favour the variations 99, 98, 97, &c. For the sake of 

 definition we shall neglect the influence of economy acting below 

 ICO, and so isolate the effects due to the mere withdrawal of 

 selection. By the conditions of our assumption, all variations 

 above 100 are eliminated, while below iCO indiscriminate varia- 

 tion is penxitted. Thus, the selective premium upon variation 

 99 being no greater than that upon 98, 98 would have as good a 

 chance of leaving offspring which would inherit and transmit this 

 variation as would 99 ; similarly, 97 would have as good a chance 

 as 98, and so on. Now there is a much greater chance of varia- 

 tions being perpetuated at or below 99, than at or above 100, 

 for at 100 the hard line of selection (or economy) is fixed, 

 while there is no corresponding line below ico. The conse- 

 quence of free intercrossing would therefore be to reduce the 

 average from 100 to 99. Simultaneously, however, with this 

 reducing process, other variations would be surviving below 99, 

 in greater numbers than above 99 ; consequently the average 

 would next become reduced to 98. There would thus be ' two 

 operations going on side by side— the one ever destroying the 

 symmetry of distribution ' round the average, ' and the other ever 

 tending to restore it.' It is evident, however, that the more the 

 average is reduced by this process of indiscriminate variation the 

 less chance there remains for its further reduction. When, for 

 instance, it falls to 90, there are numerically (though not actually, 

 because of inheritance) 89 to 9 in favour of diminution ; but 

 when it falls to 80 there are only 79 to 19 in such favour. Thus, 

 thepretically, the average would continue to diminish at a slower 

 and slower rate, until it comes to 50, where, the chances m 

 favour of increase and of diminution being equal, it would 

 remain stationary. 



"Having thus, for the sake of clearness, considered this 

 principle apait, let us now observe the effect of superadding to 

 it the influence of the economy of growth— a principle with 

 which i's action must always be associated. Briefly, as this 



^ As stated in the text, the leading idea in Mr. Darwin's suggestion was 

 that impoverished conditions of life would accentuate the principle ot 



really implii.»^ , ^^^ - „ , ^- o \ f 



Mr. George Darwin on the same subject (Nature, October 16 1873), was it 

 exhibited as an independent pnnciple. It was inarticulately wrapped up with 

 the much less significant principle of impoverished conditions. Afterward>, 

 however, Mr. Darwin expressed himself as fully persuaded of the ind<:P?": 

 dent character of the more important principle, which he was really the hrf-t 

 to perceive, alth.ugh not clearly to express. Moreover, he then thought it 

 was probably a principle of universal .-ipplication, not only ;is regards rudi- 

 mentary organ?, but also as regards degenerated structures in general. 



