CH. XL. CONCLUDING REMARKS. 435 



insects ; and in this way discoveries of great value have 

 been made. 



On the other hand great minds among us have taken up 

 the separate facts collected by specialists, and have woven 

 the whole of physical science into one grand scheme. Such 

 men as Faraday, Helmholtz, Sir W. Thomson, and Grove, 

 together with many others, have done their part in this work, 

 so that now all the various physical forces have been shown 

 to be probably phases of one great force, appearing under 

 many forms. For the future no one physical force can be 

 studied as if it existed by itself alone, for each is shown to 

 arise out of and to pass into others. Heat, electricity, mag- 

 netism, chemical affinity, motion — all are related to each 

 other, and we cannot call any one of them the ruler over the 

 rest. Like the colours on the soap-bubble, they each take 

 their turn in appearing and disappearing, according to the 

 conditions under which they arise. Their relations are 

 almost infinitely complex, and we have still much to learn 

 about them ; but the grand fact that they pass the one into 

 the other without loss of energy has been demonstrated in 

 our century ; and, under the names of ' the conservation of 

 energy,' and * the correlation of the physical forces,' is one 

 of the greatest results of modern science. 



The same tendency may be seen in the study of those 

 sciences which relate to life. Here again modern investiga- 

 tion links together the scattered observations of ages, and 

 unites them all in the theory of * evolution,* or the gradual 

 unfolding of nature ; a theory which has been worked out 

 in all its details by Herbert Spencer, one of our greatest 

 living thinkers. In astronomy, indeed, we already catch 

 a glimpse of this law in the probable formation of the 

 heavenly bodies out of gaseous star-matter ; and in the or- 



