5l8 CONCLUDING REMARKS. 



can, like these, crawl up into the shallows, or we can occasionally 

 mount at peril in a balloon ; but the utmost extent of our 

 vertical range is a distance no greater than that which we can 

 walk in a couple of hours horizontally on the earth's surface. 



The " Challenger " travelled on the voyage, from Portsmouth 

 and back to the same port, 68,690 miles ; and this distance, 

 taking into consideration the time consumed from port to port, 

 was traversed at the average pace of only four miles an hour, 

 or fast walking pace. In an express train on land the entire 

 distance could be conceived of as being accomplished in eight 

 weeks, and in about half that time at the rate at which a 

 Swallow can fly. 



If there were land all along the equator it would be possible 

 to run round the world in a train in less than three weeks. I 

 used to wonder how the main mass of the inhabitants of America 

 could have peopled the entire country down to Cape Horn, 

 from so remote a starting-point as Behring's Straits ; but a walk 

 of four miles a day would bring a man from Behring's Straits 

 to Cape Horn in about seven years, and a move of a quarter 

 of a mile a day would bring a tribe the same distance in a little 

 over a century. 



The earth, considered as a comparatively insignificant com- 

 ponent particle of the universe, may be justly compared to a 

 small isolated island on its own surface. As, in the course 

 of ages, such an island develops its own peculiar insular fauna 

 and flora, so probably on the surface of the earth alone has 

 the peculiarly complex development of the element Nitrogen 

 occurred which has resulted in the various forms of animal and 

 vegetable life. 



On the theory of evolution, it is impossible that plants or 

 animals of any advanced complexity, at all resembling those 

 existing on the earth, should exist on other planets or in other 

 solar systems. It is conceivable that very low forms of vege- 

 table life may exist on other planets and may have been by 

 some means transported to the earth : the idea is conceivable, 

 though highly improbable. But it is quite impossible that that 

 infinitely complex series of circumstances which on the earth 

 has conspired to produce from the lowest living forms a Crus- 

 tacean, for example, should have occurred elsewhere ; still less 

 is it possible that a bird or a Mammal should exist elsewhere ; 

 still more impossible again that there should be elsewhere a 

 monkey or a man. 



All these forms are quite certainly terrestrial, and terrestrial 

 only, as surely as is the Apteryx a peculiar development of 

 New Zealand alone, or the Dodo a production of the Mascarene 

 Islands only. It is even probable that protoplasm, itself, the 



