328 



♦ KNOWLEDGE ♦ 



[Nov. 30, 1883. 



deciduous forest, stretching above the helt of large-leaved 

 evergreens, which itself succeeds to the lowland palms and 

 bananas and tree-ferns of the thoroughgoing equatorial 

 plains. 



The reason for the evolution of deciduous trees is of 

 course to be found in the peculiar circumstances of the 

 circumpolar regions. In the tropics, trees and plants can 

 thrive and blossom all the year round ; and even in tem- 

 perate countries most small herbs and weeds gain by 

 keepinc; their foliage throughout the winter ; but big trees 

 in cold climates would suffer much by the tearing and 

 strewing of their leaves in winter gales, while they would 

 obtain little advantage by retaining them on the tree 

 during the long chilly season. Hence, if any tree 

 happened to possess any arrangement by which dead or 

 dying leaves could be removed without injury to the 

 permanent tissues, while, at the same time, the useful 

 materials were withdrawn into the young bark to await 

 the spring awakening, such a tree would obviously 

 enjoy an advantage in the struggle for existence, and 

 would be likely to outstrip its evergreen neighbours 

 in rigorous climates. Xow, as a matter of fact, 

 the germ of such an arrangement is found even 

 in many herbs or small shrubs, such as, for example, 

 the common pelargoniums or " scarlet geraniums " of our 

 flower-gardens. Everybody who has ever kept these 

 familiar plants in his own rooms must have noticed how 

 easily the dead leaves separate from the stem at their base, 

 by means of the swollen cellular mass where the leaf-stalk 

 joins the axis. All that the forest trees of northern 

 climates had to do, then, was just to take advantage of this 

 nascent provision, wherever it existed (mark this prior 

 necessity), and render it more fixed under the influence 

 of natural selection. But, if we may judge by the 

 actual sequel, it was not every kind of tree that 

 could thus adapt itself to the altered circumstances ; as 

 a matter of fact, the number of species among northern 

 forest-trees is very small indeed, and even out of this 

 small number a good many are conifers, like the pines 

 and yews, whose narrow, tough leaves are well fitted for 

 withstanding and battling against all the wintry breezes. 

 Still, among the conifers themselves there are a few species, 

 such as the larches, with tender delicate foliage, which have 

 also become deciduous under stress of altered conditions. 

 At the present day the large-leaved and flat-leaved ever- 

 greens are mostly confined to tropical, sub-tropical, or at 

 least warm temperate climates, and all the forest trees or 

 the circumpolar tracts are either deciduous, or else are 

 tough, leathery leaved conifers. The laurels and rhododen- 

 drons, with which we strive artificially to brighten up our 

 compai'atively leafless English winter, are either hardy 

 representatives of the warm temperate flora, or else moun- 

 tain species from southern climates, with constitutions just 

 strong enough to endure our chilly season in favoured and 

 carefully selected situations. Such evergreens have gene- 

 rally very rigid and slimy leaves to protect them — a point 

 well marked in ivy and laurel as compared with Virginia 

 creeper and English hawthorn. 



THE ZONE OF SMALL PLANETS. 



By Richard A. Proctor. 



MY attention has been called to the circumstance that 

 the theory of a shattered planet has been enter- 

 tained of late by the most eminent philosopher of our 

 age, — who, though not professedly an astronomer, has shown 

 astronomers in other matters the way to reason correctly 



about the facts which they have laboriously collected. I 

 was aware of this, but have had a reason for not dwelling 

 on the point. Indeed it was the circumstance that the 

 theory has seemed worthy of discussion by so clear-sighted 

 a reasoner which induced me to reconsider it. 



The reason which has chiefly directed attention of late 

 to the explosion theory has been that the wide extension 

 of the zone of asteroids seems inconsistent with the nebu- 

 lar hypothesis as advanced originally by Laplace. It has 

 appeared to the powerful reasoner who has lately advocated 

 the explosion theory, that, now the ring has been found to 

 contain so many members, strewn at such widely diflerent 

 distances, the theory that they are portions of a nebulous 

 ring which separated into several parts instead of collap- 

 sing into a single mass seems no longer tenable. Again 

 " did a nebulous ring break up into numerous small por- 

 tions, revolving round the Sun with ajiproximately equal 

 velocities, the annular series of them would inevitably 

 have some point of least attraction between its adjacent mem- 

 bers at which parting would take place, followed by collapse 

 of its members upon one anothertillasingle body was formed. 

 Moreover, their mean distances from the Sun could scarcely 

 difler so much that some are twice others : it could hardly 

 happen the the annular space included between their most 

 unlike mean distances would be more than 100 millions of 

 miles across, and that the space occupied Vjy the widest ex- 

 cursions would be 270 millions of miles across. Again the 

 parts of such a ring could not well have orbits much in- 

 clined to one another, or much inclined to the average 

 plane of the solar system as are sundry of the planetoid 

 orbits. Further their orbits could not difler greatly in 

 eccentricity as they di . one if not more of them to the 

 extent of cutting the oi uit of Mars. Surely no portion of 

 an outer nebulous ring could thus intrude upon the region of 

 an inner nebulous ring, at the same time that other portions 

 almost intruded upon the region of a remoter nebulous 

 ring. Once more, there could not arise any considerable 

 diflerence between the times in which the discrete portions 

 of such a ring revolved around the sun ; to the extent of 

 some being thrice others." All these traits of the plane- 

 toids, as Mr. Herbert Spencer truly says, are inconsistent 

 with the supposition of Laplace as to the origin of the zone 

 of asteroids. 



The above reasoning is so thoroughly sound and con- 

 vincing that I have ventured to quote it despite even the 

 strong reason (based on Mr. Spencer's wishes) which I had 

 had for treating this subject without any direct reference 

 to his masterly essay on the Nebular Hypothesis. Indeed 

 those who are astronomers by profession, — or at least sur- 

 veyors of starfields — take so little interest in questions of 

 this sort, that in dealing with them one must turn, of 

 necessity, to the thoughts of men like Kant, Mitehel, 

 Wright of Durham, Herbert Spencer, and others, who do 

 not find in mere observatory routine the full value of what 

 the heavens teach. 



If, as I think is demonstrable, Olbers's explosion theory 

 is absolutely inconsistent with the observed paths of the 

 asteroids and with known physical laws, and if, as I think 

 has been demonstrated above, the movements of the asteroids 

 are inconsistent with the theory that they were formed by 

 the breaking up of one of the rings of Laplace's nebular 

 hypothesis, it would seem to follow that that hypothesis is 

 either incorrect or incomplete. It has been so long my 

 own belief (and the belief of many other students of the 

 matter) that this is so, that I find nothing surprising in the 

 recognition of other and perhaps clearer evidence of the 

 fact than had before been obtained. 



What, I would ask, is known about the laws of cohe- 

 sion, of vaporous diffusion, of the interaction of the parts 



