Yet another very significant fact is seen in the distribution of comets. Though they come from all parts of the heavens, they by no means come in equal abundance from all parts of the heavens; but are far more numerous about the poles of the ecliptic than about its plane. Speaking generally, comets having orbit-planes that are highly inclined to the ecliptic, are comets having orbits of which the major axes are highly inclined to the ecliptic—comets that come from high latitudes. This is not a necessary connexion; for the planes of the orbits might be highly inclined to the ecliptic while the major axes were inclined to it very little. But in the absence of any habitually-observed relation of this kind, it may safely be concluded that, on the average, highly-inclined cometary orbits are cometary orbits with highly-inclined major axes; and that thus, a predominance of cometary orbits cutting the plane of the ecliptic at great angles, implies a predominance of cometary orbits having major axes that cut the ecliptic at great angles. Now the predominance of highly inclined cometary orbits, may be gathered from the following table, compiled by M. Arago, to which we have added a column giving the results up to a date two years later.
| Inclinations. | Number of Comets in 1831. | Number of Comets in 1853. | Number of Comets in 1855. |
| Total | 137 | 201 | 210 |
| Deg. Deg. | |||
| From 0 to 10 | 9 | 19 | 19 |
| " 10 " 20 | 13 | 18 | 19 |
| " 20 " 30 | 10 | 13 | 14 |
| " 30 " 40 | 17 | 22 | 22 |
| " 40 " 50 | 14 | 35 | 36 |
| " 50 " 60 | 23 | 27 | 29 |
| " 60 " 70 | 17 | 23 | 25 |
| " 70 " 80 | 19 | 26 | 27 |
| " 80 " 90 | 15 | 18 | 19 |
At first sight this table seems not to warrant our statement. Assuming the alleged general relation between the inclinations of cometary orbits, and the directions in space from which the comets come, the table may be thought to show that the frequency of comets increases as we progress from the plane of the ecliptic up to 45°, and then decreases up to 90°. But this apparent diminution arises from the fact that the successive zones of space rapidly diminish in their areas on approaching the poles. If we allow for this, we shall find that the excess of comets continues to increase up to the highest angles of inclination. In the table below, which, for convenience, is arranged in inverted order, we have taken as standards of comparison the area of the zone round the pole, and the number of comets it contains; and having ascertained the areas of the other zones, and the numbers of comets they should contain were comets equally distributed, we have shown how great becomes the deficiency in descending from the poles of the ecliptic to its plane.
| Between | Area of Zone. | Number of Comets, if equally distributed. | Actual Number of Comets. | Deficiency. | Relative Abundance. |
| Deg. Deg. | |||||
| 90 and 80 | 1 | 19 | 19 | 0 | 11.5 |
| 80 " 70 | 2.98 | 56.6 | 27 | 29.6 | 5.5 |
| 70 " 60 | 4.85 | 92 | 25 | 67 | 3.12 |
| 60 " 50 | 6.6 | 125 | 29 | 96 | 2.66 |
| 50 " 40 | 8.13 | 154 | 36 | 118 | 2.68 |
| 40 " 30 | 9.42 | 179 | 22 | 157 | 1.4 |
| 30 " 20 | 10.42 | 198 | 14 | 184 | 0.8 |
| 20 " 10 | 11.1 | 210 | 19 | 191 | 1.04 |
| 10 " 0 | 11.5 | 218 | 19 | 199 | 1 |
In strictness, the calculation should be made with reference, not to the plane of the ecliptic, but to the plane of the sun's equator; and this might or might not render the progression more regular. Probably, too, the progression would be made somewhat different were the calculation based, as it should be, not on the inclinations of orbit-planes, but on the inclinations of major axes. But even as it is, the result is sufficiently significant: since, though the conclusion that comets are 11·5 times more abundant about the poles of the ecliptic than about its plane, can be but a rough approximation to the truth, yet no correction of it is likely very much to change this strong contrast.
What, then, is the meaning of this fact? It has several meanings. It negatives the supposition, favoured by Laplace among others, that comets are bodies that were wandering in space, or have come from other systems; for the probabilities are infinity to one against the orbits of such wandering bodies showing any definite relation to the plane of the Solar System. For the like reason, it negatives the hypothesis of Lagrange, otherwise objectionable, that comets have resulted from planetary catastrophes analogous to that which is supposed to have produced the asteroids. It clearly shows that, instead of comets being accidental members of the Solar System, they are necessary members of it—have as distinct a structural relation to it as the planets themselves. That comets are abundant round the axis of the Solar System, and grow rarer as we approach its plane, implies that the genesis of comets has followed some law—a law in some way concerned with the genesis of the Solar System.
If we ask for any so-called final cause of this arrangement, none can be assigned: until a probable use for comets has been shown, no reason can be given why they should be thus distributed. But when we consider the question as one of physical science, we see that comets are antithetical to planets, not only in their great rarity, in their motions as indifferently direct or retrograde, in their eccentric orbits, and in the varied directions of those orbits; but we see the antithesis further marked in this, that while planets have some relation to the plane of nebular rotation, comets have some relation to the axis of nebular rotation.[K] And without attempting to explain the nature of this relation, the mere fact that such a relation exists, indicates that comets have resulted from a process of evolution—points to a past time when the matter now forming the Solar System extended to those distant regions of space which comets visit.
See, then, how differently this class of phenomena bears on the antagonistic hypotheses. To the hypothesis commonly received, comets are stumbling-blocks: why there should be hundreds (or probably thousands) of extremely rare aeriform masses rushing to and fro round the sun, it cannot say; any more than it can explain their physical constitutions, their various and eccentric movements, or their distribution. The hypothesis of evolution, on the other hand, not only allows of the general answer, that they are minor results of the genetic process; but also furnishes us with something like explanations of their several peculiarities.