xxvm.l ANALOGY 639 



In obtaining a knowledge of the stellar universe we 

 must sometimes depend upon precarious analogies. We 

 still lu>ld upon this ground the opinion, entertained by 

 Bruno as long ago as 1591, that the stars may be suns 

 attended by planets like our earth. This is the most 

 probable first assumption, and it is supported by spectrum 

 observations, which show the similarity of light derived 

 from many stars with that of the sun. But at the same 

 time we learn by the prism that there are nebulae and stars 

 in conditions widely different from anything known in our 

 system. In the course of time the analogy may perhaps 

 be restored to comparative completeness by the discovery 

 of suns in various stages of nebulous condensation. The 

 history of the evolution of our own world may be traced 

 back in bodies less developed, or traced forwards in systems 

 more advanced towards the dissipation of energy, and the 

 extinction of life. As in a great workshop, we may perhaps 

 see the material work of Creation as it has progressed 

 through thousands of millions of years. 



In speculations concerning the physical condition of 

 the planets and their satellites, we depend upon analogies 

 of a weak character. We may be said to know that the 

 moon has mountains and valleys, plains and ridges, vol- 

 canoes and streams of lava, and, in spite of the absence of 

 air and water, the rocky surface of the moon presents so 

 many familiar appearances that we do not hesitate to 

 compare them with the features of our globe. We infer 

 with high probability that Mars has polar snow and an 

 atmosphere absorbing blue rays like our o'yn; Jupiter 

 undoubtedly possesses a cloudy atmosphere, possibly not 

 unlike a magnified copy of that surrounding the earth, but 

 our tendency to adopt analogies receives a salutary correc- 

 tion in the recently discovered fact that the atmosphere of 

 Uranus contains hydrogen. 



Philosophers have not stopped at these comparatively 

 safe inferences, but have speculated on the existence of 

 living creatures in other planets. Huyghens remarked 

 that as we infer by analogy from the dissected body of a 

 dog to that of a pig and ox or other animal of the same 

 general form, and as we expect to find the same viscera, 

 th.e heart, stomach, lungs, intestines, &c., in corresponding 

 positions, so when we notice the similarity of the planets 



