360 THE POPULAR SCIENCE MONTHLY. 



had been the same as in low latitudes, became wholly changed to new 

 species after the Tertiary, while those in low latitudes remain as they 

 were in general character, and, in some cases, the identical species yet 

 survive. 



In our Southern States, for example, the flora is closely allied to, 

 and as to some species identical with, those of the Miocene of arctic 

 regions. From the Miocene back, the geological record tells of life- 

 conditions environments the same all over the world. 



So far, therefore, as geology is concerned, the evidence appears to 

 be all one way, and I think I am justified in saying that the conclusion 

 to which it points would be readily adopted, were it not for reasons 

 derived from another science. 



Astronomers say that a permanent change in the inclination of the 

 earth's axis by means of any force known to science is impossible. 

 We know, however, very little of the means by which our system was 

 brought into its present state. 



The only theory that attempts to explain, on purely mechanical 

 principles, the existence of the solar system is the nebular hypothesis 

 in some of its forms, although even that requires a self-existent entity 

 back of it. According to this hypothesis, the earth and moon were 

 once one body, which revolved, of course, on one axis. At some re- 

 mote time a separation occurred. But no force of avulsion, whether 

 the moon was merely left behind as the mass contracted, or whether, 

 as Mr. Darwin thinks, it was thrown off after the earth had become 

 solid, and pushed back to its present distance, could affect the plane 

 of rotation, or the direction of the axes. On mechanical principles, 

 the moon when it left the earth must have moved in the plane of the 

 earth's equator, and the three axes that of the earth, that of the 

 lunar orbit, and that of the moon itself must have been parallel to 

 each other. But such is not the case now. The axis of the moon is 

 inclined about 1 30', that of its orbit 5 9', and that of the earth 23 J. 

 It is evident that at some time the axes, or some of them, have under- 

 gone a change of direction. On purely mechanical principles, the 

 change did not occur before the earth and moon separated, nor at the 

 moment of separation, nor, in fact, at any time. 



Astronomy, therefore, proves too much ! It proves that the pres- 

 ent condition is not eternal ; that the earth was not created with its 

 present oblique axes in fact, that normally it was perpendicular to 

 the ecliptic ; and that, once in any position, it was impossible for it to 

 become more oblique " by any force known to science." To all of 

 which those of us who are not astronomers can only answer : " What 

 you say may all be true, but, nevertheless, the earth's axis is inclined, 

 and, if we can not show the cause an inability which extends to a 

 great many other things our business is to discover, if we can, the 

 time when it became inclined. It is not a question of possibilities, but 

 of chronology." 



