August 25, 1-887] 



NATURE 



40. 



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i 



influence would be that of continually favouring the variations 

 on the side of diminution, the effect of its presence would be 

 that of continuouily preventin_:j the average from becoming fixed 

 at 50, 40, 30, &c. In other words, the ' hard line of selection' 

 which vva-; originally placed at I03, would now becoma progres- 

 sively lowered through 90, 80, 70, &c. ; always allowing 

 indiscriminate variation below the barrier, but never above it. ^ 



"It will be understood that by 'cassation of selection from 

 changed conditions of life,' I mean a change of any kind which 

 renders the affected organ superfluous. Take, for example, the 

 exact converse of Mr. George Darwin's illustration, by supposing 

 a herd of cattle to migrate from a small tract of poor pasture to 

 a large tract of rich. Segregation would ensue, the law of 

 battle would become less severe, while variation would be 

 promoted in a cumulative manner by the increase of food. The 

 young males with shorter horns would thus have as good a 

 chance of leaving progeny as 'their longer-horned brothers,' 

 and the average length would gradually diminish as in the other 

 case. Of course, as the predisposing cause of impoverished 

 nutrition woidd now be absent, the reducing process would take 

 place at a sliwer rate. Moreover, it is to be remarked that this 

 principle differs in an important particular from that enunciated 

 by Mr. Darwin, in that it could never reduce an organ much 

 below the point at which the economy of growth, together with 

 disuse, ceases to act. For, returning to our numerical illustra- 

 tion, sippose this point t ) be 6, the average would eventually 

 become fixed at 3. 



"That the principle thus explained has a real existence we 

 may safely conclude, theoreti:al considrrations apart, from the 

 analogy afforded by our domestic races ; for nothing is more 

 certain to breeders than the fact that neglect causes degeneration, 

 even though the strain be kept isolated." 



Evidence of the wide-reaching operation of this principle 

 under Nature must be sought for in ca-es where it is impossible 

 that disuse can have had any part in the reducing'process — seeing 

 that we cannot all agree with Prof. Weismann in dismissing the 

 agency of disuse on a priori grounds of deduction from his 

 theory of germ plasma. Now, although it is not at all an easy 

 thing to find cases where the influence of the cessation of selection 

 admits of being demonstrably dissociated from the possible 

 influence of disuse, the following appear to meet the requirements 

 of the proof : — 



(i) The whole multitude of instances where recapitulative 

 phases are absent from the developmental history of an embryo 

 may stand for so many proofs of reduction without the agency of 

 disuse. For, inasmuch as none of the structures represented in 

 those phases elsewhere can ever have been of any useVo the embryo 



I from which they have disappeared, it is sufiiciently evident that 

 their obliteration can never hive been due to disuse. And, for- 

 asmuch as such structures persist in the embryos of a//;Vi/ species, 

 it appears equally evident that their reduction cannot be ascribed 

 to natural selection acting through the economy of nutrition ; 

 for, were this the case, natural selection ought to have effected 

 the reduction in the embryos of all the species. 

 (2) Even in adult organisms we meet with many structures 

 which, although of obvious use in the sen=e of affording protec- 

 tion, yet cannot be said ever to be used in the sense of being 

 actively employed, or of being employed in any way that could 

 pos-sibly lead to their structure being modified by their function. 

 Of such, for example, are the hard coverings of animals and of 

 parts of plants. It is impossible that the thickness of shells, for 

 instance, can ever have been increased by their " use " as pro- 

 tective coverings, seeing that the use is here wholly passive — is 

 not of the aotive kind which determines a greater flow of nutri- 

 tion to the part. Hence, we can only attribute the formation of 

 such structures to the unaided influence of selection. But, if so, 

 we can only attribute to the cessation of selection their subsequent 



' It is desirable to rem.irk that this numerical mode of representing the 

 principle is adopted only for the purposes of exposition. The exact point at 

 which equilibr.um would be reached in actual /cut we have no means of 

 ascertaining, since such would depend in any given case upon the original 

 force of inheKtance, or the persistence with which heredity would assert itself 

 when left entirely to itself— and of th's we have no mean; of judging. There- 

 fore, I adopt the numerical mode of rei>resenting the progressive decline of a 

 structure uider the cessation of selection merely to show that at whatever 

 point we may suppose e-iuilibrium to bereached— or a state of bal.-ince be- 

 tween heredity and indiscriminate variation to be attained — this point mu;t 

 become progressively lowered by the superadded influence of the econo ny 

 of growth. It may, however, be remarked that the initial stages of reduction 

 would probably take place more rapidly than s'lbsecjujnt stages, seeing that 

 the maxinum eflfi:iency of a structure is m.aint.ained, not only by heredity, 

 but also by the continued influence of sel:c;ion. Therefore, when the in- 

 fluence of selection is withdrawn, inliscriminate variation would rapidly 

 degrade the structure through the in'.tial stages of its reduction. 



degeneration in all cases — such as that of male cirripedes, hinder 

 parts of hermit crabs, &c. — where changed conditions of life 

 have rendered these parts no longer needful in the struggle for 

 existence. Here, indeed, economy of growth may have assisted 

 in the reduction ; but, whether or not, disuse can scarcely have 

 done so, and this is the point with which we are at present 

 concerned. 



(3) In many species of social Hymenoptera the neuter insects 

 have lost their wings. Now, as these neuter insects never have 

 progeny, it is evident that the reduction of their wings cannot 

 possibly have been due to the inherited effects of disuse. We 

 must, therefore, set it down to the cessation of selection, joined, 

 perhaps, with the economy of growth. This is a particularly 

 cogent line of proof, seeing that in some species the head, jaws, 

 and other parts of the neuters have been enlaro-ed, in order the 

 better to fit them for heavy work where strength or fighting is 

 required. Had such an enlargement been met with in the case 

 of an animal which leaves progeny, the fact might well have been 

 attributed to the inherited effects of increased use. But, as the 

 matter stands, these neuter insects are available as a demonstra- 

 tive and a double proof of the possibility both of the development 

 and the degeneration of important structures without the aid 

 either of use or of di-use. 



(4) In his essay on "Degeneration," Prof. Lankester names three 

 distinct sets of conditions as those under which the process has 

 taken place, and all these are conditions under which the cessation 

 of selection must have taken place. First, " Any new set of 

 conditions occurring to an animal which render its food and 

 safety very easily attained, seem to lead as a rule to degener- 

 ation. . . . The habit of parasitism clearly acts upon animal 

 organisation in this way. l.et the parasitic life once be secured, 

 and away go legs, jaws, eyes, and ears." In other words, so 

 soon as these organs, which were originally built up by natural 

 selection for the purpose of securing "food and safety," are 

 rendered superfluous by food and safety being otherwise secured, 

 all selective premium on their efficiency is withdrawn, and so 

 they are allowed to degenerate by indiscriminate variation. 

 Second, " Let us suppose a race of animals fitted and accustomed 

 to catch their food, and having a variety of organs to help them in 

 the chase — suppose such animals suddenly to acquire the power 

 of feeding on the carbonic acid dissolved in the water around 

 them just as green plants do. This would tend to degeneration ; 

 they would cease to hunt their food, and would bask in the suti- 

 light, taking food in by the whole surface, as plants do by their 

 leaves. . . . These vegetating animals .... show how _ a 

 degeneration of animal forms may be caused by vegetative 

 nutrition." Now, to "cease to hunt their food" is here 

 equivalent to their ceasing to be under the influence of natural 

 selection with respect to their food-hunting organs, just as in the 

 previous case. Third, " Another possible cause of degeneration 

 appears to be the indirect one of minute size. . . . The needs of 

 a very minute creature are limited as compared with those of a 

 large one, and thus we may find heart and blood-vessels, gills 

 and kidneys, besides legs and muscles, lost by the diminutive 

 degenerate descendants of a larger race." But, if " the needs of 

 a very minute creature are limited as compared with those of a 

 large one," this is the same as to say that in the "diminutive 

 descendants of a larger race" natural selection will no longer 

 operate for the maintenance of structures which have become 

 needless. In fact, in this passage Prof. Lankester comes very 

 near an express statement of the principle of the cessation of 

 selection. 



The sundry instances given in the above p.iragraphs may, I 

 hope, be held sufficient firmly to establish this principle, and to 

 show that it is one of universal application, wherever an organ or 

 a structure has ceased to be of service to the species presenting 

 it.^ Now, quite apart from the reference in which we have 



' Or, if t'aese instances are not held sufficient for this purpose, I may refer 

 to Pr.jf. Weismann's essay, wl.ere further instances are given, and also 

 supplement them with the f>llowing pass.age from my old articles in 

 Nature:— , , . ., , 



" If it be supposed that disuse is the chief cause of atrophy in wild species, 

 then it has not produced so much effect in tame species as we should antece- 

 dently expect. . . . For, supposing the cessation of selection to be here the 

 only cause at work, what degree of atrophy should we expect to find ? Be- 

 fore I turned to the valuable me.asurements given in the ' Variation of Plants 

 and Animals undei- Domestication,' I concluded (cf. Nature, vol. ix. p. 

 44t) that from 20 to 25 per cent, is the maximum of reduction we should ex- 

 pect this unassisted principle to accomplish, in the cose of natural as dis- 

 tinguished from artihciaily-bred organs. Now on calculating the average 

 afforded by each of Mr. D.arwin's tables, and then reducing the averages to 

 parts of 100, I find that the highest average decrease is 16 per cent., and the 

 lowest 5 ; the average of theaverages being rather less than 12. Only four 



