Mar. 12, 1874] 



NATURE 



361 



follow this subject out further now because it is part of a 

 larger one, which deals with the atmospheres and densities 

 of all the bodies of our system, and to discuss it at length 

 would lead me too far from the present subject ; suffice it 

 to say that the enormous atmospheres of Saturn and 

 Jupiter and the absence of a lunar atmosphere result from 

 one single cause, that cause being if I mistake not the 

 chemical constitution of the exterior parts of the solar 

 nebula as each planet was thrown off, and the subsequent 

 action on each globe. 



To come to details. Whether the walled plains are 

 really due to volcanic action or not, our authors offer 

 no opinion beyond referring to the hypothesis of Prof 

 Dana, as being the most rational. Dana suggested that, 

 as at Kilauea, the lava was simply boiling, and gradually 

 extending its boundaries from a centre, so that at last, if 

 the heat continued, a quasi-crater might be formed of any 

 extent. That the smaller craters are true craters Messrs. 

 Nasmyth and Carpenter take great pains to show, and 

 their evidence is of the closest and most satisfactory 

 kind. The woodcuts which we produce will show how 

 the central cones, which are scarcely ever absent in the 

 craters lue arc now discussing, have originated on the 

 theory advanced ; at the same time it is shown how, on 

 this idea, even when the central cone is absent or the 

 crater is filled to the brim, as in the case of Wargentin, 

 one of the most curious of lunar objects, the effects ob- 

 served may be explained. 



Mr. Nasmyth long ago illustrated the bright streaks on 

 the moon by the experiment which he here details, and a 

 very striking one it is. He took a glass globe, filled it 

 with water, and having hermetically sealed it, plunged it 

 into warm water, " the enclosed water expanding at a 

 greater rate than the glass, exerts a disruptive force on the 

 interior surface of the latter, the consequence being that 

 at the point of least resistance the globe is rent by a 

 large number of cracks diverging in every direction from 

 the focus of disruption." 



From the photograph, it is clear that the result is 

 strikingly similar to the rays which have Tycho for a 

 focus, and on the strength of this similarity the authors 

 claim this as another eft'ect of expansion due to cooling. 

 I so, however, the experiment with the glass globe is not 

 in point. But however the cracks were produced, they 

 imagine that thus having them travelling along for hun- 

 dreds of miles irrespective of surface inequality, there was 

 an up-fiow of molten matter which spread out on both 

 sides and turned the narrow crack into a broad streak. 



I trust I have said enough about this book to induce all 

 interested in physical problems to peruse it for them- 

 selves ; it is altogether an admirable production, and if 

 space permitted each picture even would merit a special 

 paragraph. 



J. Norman Lockver 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for of inions expressed 

 by /lis correspondents. No notice is tal;eti of anonymoui 

 contmnniaitions. ] 



Natural Selection and Dysteleology 

 In his reply to a criticism which appeared in Nature, 

 Prof. Struthers alluded to a question ot considerable interest 

 to evolutionists, y\i., whether or not the presence of use- 



less organs " proves too much for the argument." * The diffi- 

 culty is one often met witli, and has been well stated by Prof. 

 Huxley, thus ; — " Prof. Haeckel has invented a new and conve- 

 nient n,ime, 'Dysteleology,' for the study of the ' purposeless- 

 nesses ' which are observab'le in living organisms — such as the 

 multitudinous cases of rudimentary and apparently useless struc- 

 tures. I confess, however, that it has often appeared to me that 

 the facts of dysteleology cut two ways. If we assume, as evolu- 

 tionists in general do, that useless organs atrophy, such cases as 

 the existence of lateral rudiments of toes, in the foot of a horse, 

 place us in a dilemma. For, either these rudiments are of no 

 use to the animal — in which case, considering that the horse has 

 existed in its present form since the pliocene epoch, they surely 

 ought to have disappeared — or tliey are of some use to the ani- 

 mal, in which case they are of no use as arguments against 

 teleology," &C.+ 



Now it appears to me (as I think it must to all upon adequate 

 consideration), that the dilemma thus presented is only apparent; 

 its first-mentioned horn having no existence. In other words, 

 we can never in any single case be sure that any length of time 

 would have been sufficient to enable natural selection totally to 

 obliterate a useless organ. 



Mr. Darwin, in the "Origin" and in the "Variation," has 

 mentioned three causes which operate towards the removal of 

 useless structures. These causes are Selection, Disuse, and 

 Economy of Growth. Recently he has suggested a fourth 

 cause, which may be epitomised as tlie dwarfing influence of im- 

 poverished conditions, progressively reducing the average size of 

 the useless structure, by means ol free intercrossing. J I shall 

 endeavour to show that these causes are not sufficient to obtain 

 the complete extinction of rudimentary parts in all cases. 



Selection may be considered as applying only to those rare 

 instances in which charged conditions of life may be supposed 

 to have rendered a previously useful organ injurious ; for so 

 far as selection operates in obliterating a merely useless organ, it 

 will be more convenient, for the sake of brevity, to identify it 

 with the Economy of Growth. § Since, however, it is obvious 

 that an injurious organ must pass through the merely useless 

 stage before it becomes wholly suppressed, we may dismiss this 

 cause without further comment. 



Disuse and Economy of Growth are so much entangled in 

 their operation, that it is hopeless to find examples illustrating 

 their separate action. In so far, therefore, as we choose to dis- 

 entangle them, we must discuss the question in the abstract. 



Disuse may be left out of the question, so far as its influence 

 is due to the principle of inheritance at corresponding periods of 

 life ; for, as Mr. Darwin tersely observes, " as most organs 

 could be of no use at an early embryonic period, they would not 

 he affected by disuse ; consequently they would be preserved at 

 this stage of growth, and would remain as rudiments." il It may, 



* Nature, vol. ix p. 83. 



t " Critiques and Addresses," pp. 307-8. 



I Nature, vol. viii. pp. 432 and 505. 



§ I think, also, that if the Economy of Growth is realjy a principle inde- 

 pendent of the " more general principle," viz., the direct influence of " natu- 

 ral selection in continually trying to economise every part of the organisa- 

 tion" (Compare "Origin," 1S73, pp. 117-18), it may yet, lyithout any great 

 stretch of inference, be considered as due to the indirect influence of natu- 

 ral selection. For, the survival of the fittest demands that each individual 

 shall utilise its stock of nutriment to the best advantage ; and, this demand 

 being continued through many generations, it does not seem in itself impro- 

 bable, that It should thus at last" secure to all the members of the surviving 

 species an inherited tendency so to economise their nutritive power when 

 fre>h occasion requires— an advantageous innate temperament distinct from 

 the e.xternal cUmmativc agency of Natural Selection. Only in some such 

 way can we account 'cr the lacts of acclimatisation, in those cases where the 

 adapive changes take place immediately after the transportation of the 

 organism : also for an analogous class ol facts, such as that of the shells in 

 the same species of MoUusca, differing in their thickness upon the weather 

 and the lee sides of the Plymouth breakwater. 



il "Variation," vol li.p. 317. I may mention, also in passing, thatit seemsto 

 me not unopen to question whether disuse is the firincij'ai cAu^t: even of re- 

 duction There is no doubt that disuse causes more or less of atrophy in the 

 individual, and from this lact it is .argued, that the principle of inhentance at 

 corresponding periods of life must entail the rontmued reduction of a 

 disused part in the species. Now the only effect 01 the principle relied 

 on is that of prolonging, as it were, ihe life of the disused part over many 

 generations — ihus aflTording it an indefinite time to succumb to the conditions 

 which reduced it in the life of t'e individual. But it is necessary for the 

 validity of the infeience that it would so succumb, to show that these condi- 

 tions are the efficient causes of this reducing process in the one case, as they 

 prove themselves to be in the other. Suppose, for instance, that failure of 

 nutrition is the principal cause of atrophy under disuse, does it follow when 

 such failure has done, to all appearance, its utmost during the life of the 

 individual (as we see in diseise), that it could do any more were that life in- 

 definitely prolonged ? Of course in the case of short-lived animals, where 

 the dwarfing influences may not have time to exhaust themselves in a single 

 generation, the principle of inhf ritance at corresponding ages may be drawn 



