December 6, 1894J 



NA TURE 



gonism between the two laws of Lamarck. The first law assumes 

 that a past history of indefinite duration is powerless to create a 

 bias by which the present can be controlled ; while the second 

 assumes that the brief history of the present can readily raise a 

 bias to control the future. Edward B. Poulton. 



Oxford, December 2. 



The Homing of Limpets. 



In Nature, vol. xxxi. p. 200, Prof. .-Vinsworth Davis de- 

 scribes some observations he had made on the habits of the 

 limpet. Marked individuals were found to return from their 

 excursions, extending to a distance of some three feet, and to 

 settle down on the spot which is their permanent home. By 

 exci-ion of the tentacles in two individuals Prof. Davis was led 

 to conclude that it is not by these organs that the limpet finds 

 its way back to its own particular scar. "The sense of smell 

 then suggested itself, and it occurred to me," writes Prof. 

 Davis, "that one reason why limpets kept on their scars when 

 covered by the water was lo prevent the scent being washed 

 oflf. With a view to determine this, the space between a 

 wandering limpet and its scar and the scar was carefully washed 

 again and again with sea-water. In spite of this, the limpet in 

 question readily found its way back again." 



Last summer I had some opportunities of making observa- 

 tions at Mewps Bay, near Lulworlh, in Dorsetshire. I trust 

 that Prof. Davis will not consider a brief record of the results ot 

 these observations a case of unsportsmanlike poacning on his 

 preserves. 



The method I adopted was to remove the limpets from the 

 rock and affix them at various distances from their scars. This 

 can be done without difficulty or injury if one catches them as 

 they are moving. But one must make sure that they are iust 

 leaving or returning to their own proper homes, and are not 

 taken in the midst of a more extended peregrination, as in that 

 case their special scars cannot be noted. Failure to be 

 careful in this matter vitiated my earlier observations, which 

 are therefore excluded in the following table : — 



From the nature of the strata the removal to a distance of 

 12 inches or more generally involved taking the limpets over a 

 corner of rock. 



In most cases the individuals which failed to return :o their 

 respective scars look up new positions. In several cases when 

 they were removed to a distance of a few inches from this new 

 position they returned to it. In one case where the limpet had 

 taken up such a new position it returned thereto after having 

 been removed to its original scar. 



Observation of the limpets without such experimental re- 

 moval shows that they make their excursions in search of food 

 chiefly as the tide leaves them and when it is returning. They 

 generally seem to get back to the scar before the tide has well 

 covered it. I have watched them return over considerable dis- 

 tances. In one case ten inches, over a somewhat curved 

 course, was covered in a little under twenty minutes. In 

 another case the limpet on its return journey had to pass 

 between two other limpets, which necessitated the lifting of the 

 shell to some height so as to pass over one of these. On reach- 

 ing their scar they twist and turn about so as to fit down. 

 When they come wrong way round they rotate pretty rapidly 

 through the iSo° to get into position. The final position on 

 the scar is a constant one. One was observed to make a short 

 excursion from and to return lo its scar under stillish water. 

 As a rule they seem to remain fixed under water. 



The greatest distance I have watched a limpet reach from its 

 scar was 22 inches. But I have found limpets at a distance of 

 3 feet from their scars — that is to say, from scars on to which 

 they fitted perfectly. This was on a large fiat surface. 



When they move, the tentacles are projected out beyond the 

 shell, and keep on touching and slightly adhering to the rock. 

 On reaching the scar they carefully feel round it with the tent- 

 acles. I am disposed to question the results of Prof. Davis's 

 experiments on the removal of the tentacles. But further 



NO. 



I3IO, VOL. 51] 



observations and experiments are needed to settle the point. 

 I understand that Prof. Davis is now at work upon the subject. 

 .\n injury to the edge of the shell seems to be repaired with 

 whitish shell-material in the course of about ten days. And 

 when a new position is taken up to which the shape of the shell 

 is not suited, there appears lo be a tendency for the shell to 

 accommodate itself to the uneven surface of new growth along 

 the edges. But this again is a matter on which further obser- 

 vation and experiment are required. 



C. Lloyd Morgan. 



Gravitation. 



The nature of my suggestion {vide Nature November 15, 

 p. 57) is simply this: — A phenomenon of adhesion between 

 solids immersed in a tensile liquid presents itself. The explana- 

 tion ofi'ered (as I understand it) suggests that whether the bodies 

 are attracted at long or short distances, will be a question 

 entirely of the extension in the stressed medium of the modified 

 layer. If this explanation be a correct one, or if any explana- 

 tion involving a reaction between a modified layer (whether 

 condensed or rarefied) and a tensile liquid will account for the 

 phenomenon, then I say the experiment is a suggestive one as 

 regards gravitation. 



How far the modified layer will extend depends upon the l.iw 

 according to which the stress is distributed in the medium. In 

 the case of matter acting upon matter at molecular distances we 

 have reason to believe that the decrement of the stress is a rapid 

 one. We possess no such knowledge when matter and ether 

 alone are involved, and until we know how a modification of 

 the ether around matter would display itself to our observation, 

 I do not think the possibility of a remotely extended modifica- 

 tion can be denied. Gravitation might be the sole resultant 

 phenomenon affecting our senses. J. Joly. 



Trinity College, Dublin. 



The Ratio of the Specific Heats of Gases. 



I REMEMBER that in the discussion of eighteen years ago it 

 was understood that you could get I '4 for the ratio, if the- 

 molecules had each five degrees ol freedom only— if they were, 

 for instance, perfectly smooth, elastic spheroids. Probably the 

 ultimate source of our knowledge in this respect was Boltzmann'-.^ 

 paper, to which Mr. Bryan refers us. The difficulty at the time- 

 seemed to be mainly one of faith. One could not believe that 

 the molecules were solid elastic bodies, however useful the dis. 

 cussion of such bodies might be in defining a limiting case. As. 

 the white posts along a road are put to show you where yon 

 should not go, not where you should go. It was further sup- 

 posed, perhaps without sufficient reason, that the phenomena 

 of the spectroscope required us to attribute many degrees of 

 freedom to the molecules. 



I hope Mr. Bryan will, as I have no doubt he can, develop- 

 his theory that all these phenomena can be accounted for by: 

 the electromagnetic theory of light, without attributing to thir 

 molecules more than five degrees of freedom. We have to ex- 

 plain, as it seems to me, how the ether will assume ditTerent sel-^ 

 of vibrations according to the shape of the bodies in contact 

 with it. S. H. BURBURY. 



I New Square, Lincoln's Inn. 



An Observation on Moths. 



I THINK Dr. L. C. Jones (No. 130S, p. 79) has missed the- 

 true reason of the unexpanding wings of his moths liberateil 

 from the pupa-case before the struggles of the inmate had splii 

 the skin, and freed them in the ordinary course. 



What was missing to them was the pressure in the act of 

 emergence, which at one and the same time expels a discharge 

 of superfluous humours from the abdomen, and forces the vital 

 fluids through the folded and crumpled wings. Special extra 

 provision is made for this, in the flask-shaped cocoons ol 

 Saturnia Pavania- minor, for example, and if the pupa be 

 taken out of this, and allowed to emerge at full matuiity, it is 

 always an abortion with heavy, overloaded abdomen, and 

 wings that never expand. Every collector, also, who has bred 

 the earth-burying sphinxes — Sphinx Liguslri, for example — 

 knows how often they emerge in this condition, either through 

 not being supplied with soil of the needful tenacity, or from the 

 difficulty of keeping it of the natural degree of moisture. 



