Page:Popular Science Monthly Volume 1.djvu/779

Rh modified for the purpose. There was a beautiful adaptation of means to an end, and a mutual interdependence between the plant and the animal. Prof. Riley then explained succinctly how, on Darwinian grounds, even this perfect adaptation was doubtless brought about by slow degrees. He alluded, in closing, to a practical phase of the subject. The plant and its fructifier are inseparable under natural conditions, and the fructifier is found in the native home of the plant. In the more northerly parts of the United States and in Europe, where our yuccas have been introduced, and are cultivated for their showy blossoms, the insect does not exist, and consequently the yuccas never produce seed in those places. The larva of pronuba eats through the yucca-capsule in which it fed, enters the ground, and hibernates there in an oval silken cocoon. In this state the insect may easily be sent by mail from one part of the world to another, and our own florists may, by introducing it, soon have the satisfaction of seeing their American yuccas produce seed, from which new plants can be grown.

This paper was extremely interesting to every one present, and those who discussed it pronounced it in every respect admirable. Dr. Asa Gray, than whom no one in the country is better able to form a sound opinion upon such a subject, complimented Prof. Riley on his discovery, and the lucidity of his explanation before the section.

Binary Stars.—The same journal gives a sketch of a paper on this subject by Prof. Daniel Kirkwood, of which the following is a summary:

At the meeting of the Royal Astronomical Society of London, on May 18, 1872, it was announced by Mr. Wilson that a discussion of all the observations of the double star Castor, from 1719 to the present time, had led to the remarkable conclusion that the components are moving in hyperbolas, and, consequently, that their mutual relations as members of a system are but temporary. The fact, if confirmed, will be regarded with great interest, and its discovery will doubtless be followed by a minute and vigilant scrutiny of other binary systems. But, while such a relation as that discovered by Mr. Wilson had not been previously suspected, its existence was certainly not altogether improbable. As the sun in his progressive motion through space compels such cometary matter as may come within the sphere of its influence to move about him in parabolas or hyperbolas, so two bodies of the same order of magnitude may be brought by their proper motions within such proximity that their mutual attraction shall cause each to move about the other in a hyperbolic orbit. Such instances, however, would seem to be exceptions to the general rule, as the motion of most binary stars is undoubtedly elliptic. (This fact has been explained in 1864 by the author on the basis of the nebular hypothesis.)

The components of Castor are of the magnitudes three and three and a half respectively. If we suppose that each before the epoch of their physical connection was the centre of a planetary system, the results of perturbation must have been extremely disastrous. The two stars were at their least distance in 1858.

This alleged discovery of a temporary physical connection between two fixed stars suggests a number of interesting inquiries. In the infinitely varied and complicated movements of the sidereal systems, different bodies may be brought into such juxtaposition as to change not only the direction of their motions, but also the orbits of their dependent planets. Some stars, at the rate of motion indicated by the spectroscope, would pass over an interval equal to that which separates us from the nearest neighboring systems in 20,000 years. In view of these facts, the conjecture of Poisson, that the temperature of the earth's surface at different epochs has depended upon the high or low temperature of the portions of space through which the solar system has passed, may not be wholly improbable.

A possible origin of binary systems is also indicated by Mr. Wilson's discovery. The cometary eccentricity of the orbits of these bodies is well known. In some cases the estimated distance between the components at the time of their periastral passage is less than half the radius of the earth's orbit. Now, if at the epoch of the