Dec. 14, 1883.] 



♦ KNOWLEDGE ♦ 



357 



any individual meteor would be to cai^e its mean distance 

 to oscillate on either side of that particular distance which 

 corresponded to a period synchronising (hiniply) with 

 Jupiter's. But considering the millions of meteors form- 

 ing the system or stream, and that these would all have 

 periods oscLUatiug through this mean value, it is evident 

 that at any given moment there would be multitudes of 

 meteors moving in that period, or at the mean distance 

 corresponding to one of the gaps in the asteroidal zone. 



But now notice that the meteors thus passing outside 

 and inside their proper mean distances would enter regions 

 traversed by other meteor streams, and that, the members 

 of such streams being counted by millions of millions, 

 collisions could hardly fail to occur by myriads. Every 

 collision would in efl'ect be the withdrawal of a meteoric 

 mass, small or great, which until then had had a period 

 synchronising with Jupiter's ; for in whatever way a 

 collision occurred the mass resulting from the collision 

 would have a period diflering from that of either of the 

 colliding masses. 



Such processes would, in general, be part of the long 

 series of processes leading to the eventual formation of a 

 single planet. But just within the orbit of the mightiest 

 of all the planets, Jupiter, and where the velocities due to 

 solar influence begin to be great (this region being com 

 paratively near to the sun) the combined disturbing 

 influences would be better able than anywhere else through- 

 out the solar system to prevent the formation of a single 

 planet, and would lead, instead, to the formation of a zone, 

 or rather a system of zones of small aggregations. These 

 zones would be separated by gaps, either wholly or almost 

 wholly free, corresponding to the distances at which 

 Jupiter's disturbing influence would be most effectively 

 exerted. 



The circumstance that such gaps exist precisely where 

 on this theory they would exist affords, certainly, strong 

 evidence in favour of the theory. 



But when we turn to the seventh point noted above the 

 evidence seems to become decisive. 



In the Saturnian ring-system we have a zone of multi- 

 tudinous small bodies, travelling each in its own special 

 orbit around the central ruling orb, Saturn. Whether we 

 have here a region where other satellites besides those 

 already formed around Saturn are in process of formation, 

 or a region which will be, for all time, occupied by zones of 

 tiny bodies, we need not here stop to inquire. It is sufli- 

 cient that we have in the Saturnian rings, actually present 

 before our eyes, a stage in the development of a system 

 resembling the planetary family. Of course, it might 

 be urged that here we have evidence of the bursting 

 of two or three Saturnian satellites which formerly 

 travelled around the planet where now we see two 

 or three rings ; but I hardly think this theory will be 

 advanced, or (if advanced) successfully maintained. 

 Setting it aside, we find in the Saturnian rings the ana- 

 logue of the asteroidal system. There, on a small scale, 

 are precisely such gaps as we have in the asteroidal system 

 on a larger scale. The gaps are not quite free from small 

 satellites, for if they were they would be as black as the 

 shadow of the planet on the ring, and they are not black, 

 but brown or dark grey. They are regions whence the 

 small satellites have been almost wholly swept away. If 

 the same sort of action can be recognised here that I have 

 considered above in the case of the asteroidal zone, no 

 manner of doubt should, I imagine, be entertained as to 

 the validity of that interprctatiou of the gaps in the 

 asteroidal system. But the very same cause has been 

 recognised. Professor Kirkwood, of Bloomington, Indiana, 

 with that keenness to detect such neat proofs which charac- 



terises him, and led me long since to call him the Kepler 

 of modern astronomy, has shown that the gaps in the 

 Saturnian ring system lie precisely where the perturbing 

 action of the neighbouring satellites would l>e most 

 efficient. 



DARWIN ON INSTINCT. 



" A/F ^^TI-P-'^^' ^'^Ti let the strongest live and the 

 IVJ. weakest die." If not absolutely the last word, 

 this is at least the last published utterance of the author of 

 " The Origin of Species " and " The Descent of Man." In 

 these ten words Darwin has expressed the conclusion of the 

 whole matter, the ultimate result of a lifetime's patient and 

 ceaseless research into the laws which govern terrestrial 

 life. They are the concluding words of a paper by the 

 late Mr. Darwin which his most faithful and loving disciple, 

 Mr. G. J. Romanes, read recently at the Linnean Society 

 This paper, or " Posthumous Essay," as Mr. Romanes calls 

 it, is the full text of a part of Dar-n-in's chapter on Instinct 

 written for the " Origin of Species," but afterwards sup- 

 pressed for the sake of condensation. This and other papers 

 on psychological subjects, to the elucidation of which from 

 a physiological standpoint Mr. Romanes has devoted 

 himself, Darwin gave to his disciple and friend, with 

 the request that the latter should publish any parts of 

 them that he chose in his works on mental evolution. Mr. 

 Romanes, however, after Darwin's death, naturally did not 

 feel himself at liberty to appropriate and incorporate in his 

 works, as his own, so much material carefully collected and 

 sifted by the master's hand, and the value of which had 

 become enhanced. He, therefore, rightly decided to give 

 the last, as Darwin himself gave the first, of the " Origin 

 of Species " to the Linnean Society. The paper itself 

 appears as an appendix to his work on " Mental Evolution 

 in Animals," published by Messrs. Kegan Paul k Co. But 

 that work contains much more of Darwin than the 

 appendix. He had collected many notes bearing on the 

 subject of instinct over and above the suppressed essay, and 

 most of these notes Mr. Romanes has introduced into the 

 text of his own work, always taking care to indicate 

 their authorship, and giving, moreover, a special index to 

 the passages. 



In the essay before us, Darwin endeavours to throw 

 light upon the origin of four manifestations of what is 

 usually called instinct :—(l) Migration; (2) Instinctive 

 fear ; (3) Niditication and Habitation ; (4) Habitations of 

 mammals ; and after a few miscellaneous remarks on the 

 subject of instinct and on cases of special difficulty, he 

 states his conclusion. The subject of the migration of 

 birds is one of the most curious and complex in connection 

 with what we call the instinctive habits of animals. But 

 the tendency is not confined to birds, though Ln that class 

 it seems most marked and developed. As usual, through- 

 out this essay Darwin piles instance on instance to shed 

 light on all sides of the special point under discussion. 

 After adducing various instances to prove how widespreatl 

 the migratory instinct is, and how in seeking to satisfy it 

 birds of the most delicate structure will cross hundreds of 

 miles of sea, he endeavours to show how the more remark- 

 able migrations could possibly have originated. He 

 says ; — 



Take the case of a bird being driven each year by cold or want 

 of food, slowly to travel northward, as is tlio case with some birds, 

 and in tirao wo may well believe that this compulsory travcUing 

 would become an instinctive passion, as with the sheep in Spain 

 [referring to an instance ho gives]. Now daring the long course of 

 ages, let valleys become converted into estuaries, and then into 

 wider and wider arms of the sea; and still I can well believe that 



