FUTUKE HABITABILITY OF THE EARTH CHAMBERLHST. 387 



tions of a revolutionary character have disclosed sources of energy in 

 radioactivity of an extremely high order. In the light of these dis- 

 closures, the forecast of the sun's probable power to energize suffi- 

 ciently the activities of its own atmosphere and of ours, and to warm 

 the earth adequately, is raised to an indeterminate order of mag- 

 nitude. 



We thus find grounds for a complacent prophecy of the earth's 

 future habitability. This prophecy seems to me to gain strength 

 from its appeal to a series of reciprocities between land and sea, be- 

 tween earth and air, and between the planet and the solar center that 

 seem to have been potent in all the history of the earth from its gen- 

 esis to the present time. 



But if traditional fears from these domestic sources be dismissed, 

 may we hold ourselves free from impending dangers from the heavens 

 without ? 



So far as present knowledge goes, one tangible possibility of dis- 

 aster from without our system seems to be contingent ; the possibility 

 of collision with some celestial body, or, what is many times more 

 contingent, such a close approach to some massive celestial body as 

 to lead to serious disruptive effects. Within the solar system, the 

 harmonies of movement already established are of such an order as 

 to give assurance against disaster for incalculable ages. Comets do 

 indeed pursue courses that may, theoretically at least, bring about 

 collision, but comets do not usually appear to possess masses sufficient 

 to work disaster to the life of the earth, as a whole, whatever local 

 catastrophies might be suffered at the points of impact. The motions 

 of the stars trend in diverse directions, so that collisions and close 

 approaches between them seem, theoretically, possible and probable, 

 if not inevitable. There are in the heavens also many nebula? and 

 perhaps other forms of scattered matter, and there are doubtless also 

 dark bodies, all of which offer possibilities of collision. The appear- 

 ance of new stars flashing out suddenly and then gradually dying 

 away suggests the actual occurrence of collisions or disastrous ap- 

 proaches. Though these seem destructive on their face, and are so, 

 no doubt, it has been held that the close approach of suns is one 

 of the regenerative processes of the heavens, and that by it old plane- 

 tary systems are dispersed and new systems brought into being. One 

 phase of the planetesimal hypothesis is built on this conception. It 

 postulates the close approach of some massive body to our ancestral 

 sun long ago, and that by this approach the sun's former planetary 

 system, if it had one, as is thought probable, was dispersed, and at the 

 same time the matter for the present planetary system was thrown 

 out into a nebulous orbital state by the explosive power resident in 

 the sun aided by the differential pull of the great body that was 



