April (), 1 8 74 J 



NATURE 



441 



nate variation is permitteJ. Tluis, the selective premium upon 

 variation 99 being no greater than tliat upon 98, 98 would have 

 as good a chance of leaviuij 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 thus a much 

 greater chance of variations 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 

 ICX). The consequence of free intercrossing would therefore be 

 to reduce the average from 100 1099. 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 destroy- 

 ing 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 dimi- 

 nution ; but, when it falls to So, there are only 79 to 19 in such 

 favour. Thu5 (theoielically) the average would continue to 

 diminish at a slower and slower rate, until it comes to 50, where, 

 the chances in favour of increase and of diminution being equal, 

 it would remain stationary. 



Having thus, for the sake of clearness, considered this prin- 

 ciple apart, let us now observe the effect of superadding to it the 

 influence of the Kcononiy of Growth — a principle with which its 

 action must always be associated. Briefly, as this influence 

 would be that of coniinually favouring the variations on the side 

 of diminution, the effect of its presence would be that of con- 

 tinuously preventing the average from becoming fixed at 50, 40, 

 30, &c. In other words, the ''hard line of Selection," which 

 was originally placed at 100, would now become progressively 

 lowered through 90, So, 70, &c. ; always allowing indiscriminate 

 variatioir below tl'e barrier, but never above it. 



It will be understood that by "cessation of selection from 

 changed conditions of life" I mean a change of niiy kind which 

 renders the »fl'ected o'rgan superfluous. Take, for example, the 

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

 ing 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 pro. 

 moted 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. (Jf course, as the predisposing cause of impoverished 

 nutrition would now be absent, the reducing process would take 

 place at a slower 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 

 muclr below the point at which the Economy of Growth (toge- 

 ther with Disuse) ceases to act. For, returning to our numerical 

 illustration, suppose this point to be 6, the average would even- 

 tually become fixed at 3. 



That the principle thus explained has a real existence we may 

 safely conclude, theoretical considerations 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. It will be observed that, if 

 this principle has a real existence, it is of considerable importance, 

 theoretically, since it must act, to a greater or less extent, in 

 all cases where Disuse and the Economy of Growth are in opera- 

 tion ; and although in the initial stages of r-eduction, when the 

 pirrchase, so to speak, of the last-named jrinciple is great, its 

 influence would be eoniparatively tiiviil, this influence would be 

 more and more felt the smaller the organ became — i.e., the nearer 

 the point at which the Economy of Growth ceases to act. The 

 Cessation of Selection should therefore be regarded as a reducing 

 cause, which co-operates with other reducing causes in all cases, 

 and which is of special importance as an accelerating agent wdien 

 the influence of the latter becomes feeble. 



GliORC.r. J. ROMAXF.S 



Lakes with two Outfalls 



On June 22, 1S63, the late Captain Speke published his 

 map, giving (on native authority) /i'«i- outlets from Lake Victoria 

 Nyanza, converging to cit€ valley or water-flow— the Nile. 



On June 27 and on July 20 I wrote to the AthemTum :— " I 

 think that this native information will prove to be erroneous ; " 

 that I thought "that no lake can have more than one outlet ;" 

 and I added, "j\lay I lay the question as to the matter of fact 

 before the readers of the AlluiuTum ? " In reply, the Black Loch 

 in Dumfriesshire was stated to have tvo outlets to ttvo distinct 

 valleys or water-flows— one to the Nith, the other to the Ayr. 

 The Loch, however, has but one outlet, and that aiiificial. The 

 water-parting has been cut through by man — a mill-lead made to 

 Lord Bute's Borland mill, and the one outlet is an iion sluice in 

 a stone dam. All this is beautifully shown in Sir Henry James's 

 adirrirable 25-inch Ordnance Maps. 



Dr. Bryce (" Geology of Arrair," 3rd edition, p. 3) says that 

 Loch-an- Davie has two outlets to two different valleys. It has, 

 however, but one outlet, to the south— to Glen lorsa, as I stated 

 in the AlJieiiiTuni, July 22, 1865. The new inch Ordnance Map 

 of Arran gives one outlet, but unfortunately to the north, instead 

 of to the south. I will not refer to my letter on the two outlets 

 to two valleys from the Norwegian Lesje.'.kaugen Lake, which 

 you did me the honour to publish iast September, and with 

 which Prof. Stanley Jevons agreed. But I quote the above cases 

 to show that even the highest authorities make mistakes as re- 

 gards lakes and their outlets. I cannot, however, suppose any 

 mistake in Prof. Bell's account of the two outlets to two valleys 

 from Shoal Lake, published m Nature, vol. ix. p. 363, by 

 Prof. Dawson. I would then, in deference to these authorities, 

 modify my dictum by saying, that if by a rare possibUrty a lake 

 may be found to exist on a water-parting having at opposite ends 

 two outlets to two different valleys, I should still doubt the possi- 

 bility of a lake at its one lower end having a multiplicity of out- 

 lets converging to one valley or water-flow, as in the case of the 

 Victoria Nyanza. And this owing to the extreme improbability 

 that the erosion at each outlet should continue at precisely the 

 same rate. 



The outlet of every lake in the wide, wide world is always 

 being lowered from erosion, as are valleys themselves. Valleys 

 exist only in the dissolution of hills. They are mere water-flows. 

 They are the perpetually changing effects of atmospheric disin- 

 tegration, and the erosion of rain and rivers, and consequently 

 every water-parting is a valley-parting. 



Alresford, March 14 George Greenwood 



A Beech pierced by a Thorn Plant 



On the road from this to Belfast there is a thorn hedge with 

 beech trees at interi'als, and thorn plants have grov/n right 

 through the middle of the trunks of two of the beeches. I do 

 not know whether this is sufficiently uncommon to be worth 

 mentioning in Nature. Joseph John Murphy 



Old Forge, Dunraurry, co. Antrim 



KINETIC THEORY OF THE DISSIPATION OF 



ENERGY 



T N abstract dynamics an instantaneous reversal of 

 -'■ the motion of every moving particle of a system 

 causes the system to move backwards, each particle of it 

 along its old path, and at the same speed as before when 

 again in the same position — that is to say, in mathemati- 

 cal language, any solution remains a solution whin /is 

 changed into —i. In physical dynamics, this simple and 

 perfect reversibility fails on account of forces depending 

 on friction of solids ; iinperfect fluidity of fluids ; imperfect 

 elasticity of solids; inr qualities of temperature and con- 

 sequent conduction of heat produced by stresses in solids 

 and fluids ; imperfect magnetic retentiveness ; residual 

 electric polarisation of dielectrics ; generation of heat by 

 electric currents induced by motion ; diffusion of fluids, 

 solution of solids in fluids, and other chemical changes ; 

 and absorption of radiant heat and light. Consideration 

 of these agencies in connection with the all-pervading 

 law of the conservation of energy proved for them by 

 Joule, led me twenty-three years ago to the theory of the 

 dissipation of energy, which I communicated first to the 

 Royal Society of Edinburgh in 1852, in a paper entitled 



