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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



bility of any unteleological (or unadapted) forms 

 existing in a systematically-connected whole. 



From this point of view the evolution of the 

 Cosmos is seen to be, as indeed we should have 

 expected, entirely analogous to the evolution of 

 all the kingdoms of Nature. Just as in biology, 

 for example, adaptation to environment is at- 

 tained only indirectly through a weeding - out 

 process, which consists in the elimination of or- 

 ganisms that are incapable of existing : so in 

 the mechanism of the heavens does the law of 

 gravitation, by indirect selection, work adapta- 

 tion between the different members of a system, 

 by eliminating such members as are incon- 

 sistent with the rest. The perturbations aris- 

 ing out of the mutual attraction of planets 

 have indirectly— that is, by eliminating the 

 greater part of the earlier comites of the sun 

 — made selection of that small number of plan- 

 ets which now exist ; and our present planets 

 are able to continue in existence, in spite of 

 their mutual gravitation, because of the incom- 

 mensurability of their periodic times. Nature 

 acts after the manner of the engraver on wood, 

 who makes his engraving in relief only indirectly, 

 by cutting away the face of the wood between 

 his lines. 



Cosmical problems are to be solved only by 

 concluding from phenomena offered to us in ex- 

 perience to phenomena in the past. But now 

 perturbations are familiar phenomena in the plan- 

 etary system, and their only effects consist in di- 

 verting the planets a little out of their regular 

 course. But that they might also exert the higher 

 power attributed to them in the preceding para- 

 graph — namely, the elimination of cosmic bodies 

 from the system — we see from the comets, whose 

 orbits not seldom are entirely transformed as the 

 result of perturbations. 



It has been demonstrated by Newton that, 

 under the quadratic law of attraction, the tracks 

 of the planets can only be conic sections with the 

 point of attraction for focus, their forms being, 

 according to the ratio of the force of gravity to 

 the centrifugal force, circles, ellipses of different 

 lengths, parabolas, or hyperbolas. Actual elimi- 

 nation of planetary bodies, therefore, could take 

 place only on this relation of gravitation to cen- 

 trifugal force being so materially altered that 

 either, the force of gravitation having increased, 

 a spiral motion toward the centre of attraction 

 was set up, or else that, the centrifugal force 

 having been considerably increased by perturba- 

 tion, the original circular track was converted 

 into a not-closed orbit, whether parabolic or hy- 



perbolic. But all those planets whose tracks 

 were converted only into elongated ellipses re- 

 mained in the system. 



Yet such planets do not exist. Only comets 

 and meteorite-showers move in such elongated 

 ellipses — that is to say, those very bodies which, 

 as we have said, it is incumbent on us to embrace 

 within the nebular hypothesis. But not only is 

 it now certain that the meteorites are fragments 

 that once belonged to great planetary bodies, but 

 Schiaparelli has even demonstrated the connec- 

 tion between comets and meteorites ; while, from 

 Zollner's researches it appears to be highly prob- 

 able that comets are only meteorites of vaporiza- 

 ble matter. 



It only remains, therefore, to prove that plan- 

 etary bodies, which were by primordial perturba- 

 tions forced into elongated orbits, must, in con- 

 sequence, have progressed more rapidly toward 

 disintegration than those which were forced only 

 a little out of a circular track. In investigating 

 this subject we shall derive material assistance 

 from the comparative astronomy of the planetary 

 system, since in our planets and moons, whose 

 conditions represent various phases of a like pro- 

 cess of development, the ultimate phase — that of 

 the meteorite-showers — is already more or less 

 plainly typified. But I will Dot discuss this 

 point here, having considered it fully in another 

 place. 1 



According to the nebular hypothesis, the mat- 

 ter constituting our system was at one time dif- 

 fused beyond the orbit of Neptune ; and it has 

 been computed that matter so attenuated as this 

 must have been could not have exceeded in den- 

 sity the ten-millionth part of the density of the 

 lightest gas known — namely, hydrogen. Such 

 enormous attenuation as this — an attenuation 

 presupposing a development of heat that is hard- 

 ly imaginable — is in itself enough to force us to 

 the conclusion that there was at one time much 

 more matter within the space circumscribed by 

 Neptune's orbit than exists there now ; and that, 

 consequently, the primordial nebula was very, 

 very much less attenuated than it was if the esti- 

 mate above were correct. For this reason the 

 meteor streams and comets must be for us wel- 

 come phenomena ; for if they once existed in 

 interplanetary space in the state of dissolution, 

 and passed beyond its limits only in consequence 

 of later perturbations, then they must bear some 



1 See " Dcr Kampf cms Dasein am Ilitnmel : Versuch 

 einer Philosophie der Astronomie" ("The Struggle for 

 Existence in the Heavens : An Attempt at a Philosophy 

 of Astronomy "). 



