1874. | The Past History of our Moon. 307 
the youngest member of the solar system, while the tiniest 
members of the asteroid family, if not the oldest in reality, 
are the oldest to which the telescope has introduced us. 
Jupiter and Saturn come next to the sun in youth ; they are 
still passing through the earliest stages of planetary ex- 
istence, even if we ought not rather to adopt that theory 
of their condition which regards them as subordinate suns, 
helping the central sun to support life on the satellites which 
circle around them. Uranus and Neptune are in a later 
stage, and perchance when telescopes have been constructed 
large enough to study these planets with advantage, we may 
learn something of that stage, interesting as being inter- 
mediate to the stages through which our earth and Venus 
on the one hand, and the giant brothers Jupiter and Saturn 
on the other, are at present passing. Atter our earth and 
Venus, which are probably at about the same stage of 
planetary development (though owing to the difference in 
their position they may not be equally adapted for the 
support of life) we come to Mars and Mercury, both of which 
must be regarded as in all probality much more advanced 
* and in a sense more aged than the earth on which we live. 
In a similar sense,—even as an ephemeron is more aged 
after a few hours of existence than a man after as many 
years,—the small planet which we call “‘ our moon” may be 
described as in the very decrepitude of planetary existence, 
nay (some prefer to think), as even absolutely dead, though its 
lifeless body still continues to advance upon its accustomed 
orbit, and to obey the law of universal attraction. 
Considerations such as these give singular interest to the 
discussion of the past history of our moon, though they add to 
the difficulty of interpreting the problems she presents to us. 
For we have manifestly to differentiate between the effects due 
to the moon’s relative smaliness on the one hand, and those 
due to her great age on the other. If we could believe the 
moon to be an orb which simply represents the condition to 
which our earth will one day attain, we could study her 
peculiarities of appearance with some hope of understanding 
how they had been brought about, as well as of learning from 
such study the future history of ourown earth. But clearly 
the moon has had another history than our earth. Her 
relative smallness has led to relations such as the earth 
never has presented and never will present. If our earth is, 
as astronomers and physicists believe, to grow dead and cold, 
all life perishing trom her surface, it is tolerably clear 
from what we already know of her history that the ap- 
‘pearance she will present in her decrepitude will be utterly 
