230 Original Articles. [April, 



tion of each. Previously to this, however, it should be said, that much 

 more is known of some than of others, proximity to the sun in the 

 one case and distance from him in the other, rendering observations 

 on the extreme members difficult and uncertain. But, without in- 

 quiring into the exact limits of fair analogy, it will be readily granted 

 that where, in case of a strong presumption existing in favour of an 

 adaptation productive of some degree of similarity throughout the 

 members of a related series, such an adaptation can be clearly proved 

 in respect of two or more, the presumption is immensely strengthened 

 that it extends to all. With respect to Mars, however, the proofs for 

 this are complete ; and in the case of Jupiter and other planets, as 

 much so as the difficulty of observation can render them. 



Moreover, where actual experience cannot be obtained, the want of 

 it is no argument against a given probability. It is quite conceivable, 

 for instance, that the animals obtained by Torell, Milne-Edwards, Dr. 

 Wallich, and Lieutenant Brooke, from the black depths of an almost 

 fathomless sea, under a pressure which not long since was thought 

 to be utterly incompatible with life, might, if endowed with reason, 

 and arguing from their limited experience only, question the possi- 

 bility of creatures organized like themselves existing in such widely 

 different conditions as those of their own station on the one hand, and 

 such as are supplied by shallow water and abundant sun-light on the 

 other. And yet, we now know that such reasoning would be incon- 

 clusive, because in these opposite circumstances creatures with pre- 

 cisely similar organizations do live. 



To come now to what is known concerning the physical constitu- 

 tion of different members of our system. — The Moon, from being 

 nearest to us, is of course best known ; the charts of Boer and Madler, 

 and more especially the beautiful photographs by Mr. De La Bue, and 

 models and drawings by Mr. Nasmyth, having made the part that is 

 turned towards the Earth familiar to most people. So far as our pre- 

 sent means of judging extend, this part cannot be inhabited, because 

 many circumstances prove it to be devoid of an atmosphere. The 

 chief of these are the want of appreciable refraction, the sudden ex- 

 tinction of small stars by her limb, and the absence of any atmo- 

 spheric lines whatever in the spectroscope ; although by this latter 

 method very small portions of her surface can be separately examined. 

 Thus, Mr. Huggins and Dr. Miller were able to observe parts equal only 

 to "0000346 of the hemisphere, or about one-third the size of Tycho, a 

 lunar mountain some 19,000 feet high. But this uninhabitability of 

 the Moon, even if it could be shown to extend to her entire surface, so 

 far from weakening the opinion that the primary planets are tenanted 

 serves only to strengthen it, because just those respects in which they 

 differ from her are the ones which are known to be essential to the 

 maintenance of life, and, therefore, the existence of such differences 

 renders more probable the existence of the ends to which they are 

 known specially to minister. But from the uninhabitability of the 

 proximate surface of the Moon, we are not warranted in concluding 

 anything concerning the remote hemisphere, the physical conditions 

 of which, with the exception of those of the small portions seen during 

 her librations, are utterly unknown ; and that it may possess the con- 



