4 HABIT AND INTELLIGENCE. 



to Newton's law of gravitation, it has been shown that the 

 laws of force and motion apply, not to masses of matter 

 only, but also to atoms : it has been shown that those 

 laws are true on all scales of magnitude, from a star to 

 an atom. 

 Summary. Briefly to recapitulate the substance of the last few 

 paragraphs : — Astronomy has proved the unity of natural 

 law through space, and geology through time ; while the 

 science of heat, and we raay add those of light and elec- 

 tricity, have shown the same to be true on all scales. 



This tendency to identify laws which at first appeared to 



have nothing in common, and thus to establish a wide and 



comprehensive unity of law, is not yet so conspicuous in 



the sciences of life and mind as in the inorganic sciences. 



The same I bcHeve, however, that the only reason for this is the 



tendeucyis comparative backwardness of the sciences of life and mind, 



discemibJe ^ 



ill the which is a necessary result of the greater complexity of 

 life and ° their subject-matter. But even in them, there is a manifest 

 mind. tendency to break down apparent distinctions, and to 

 establish real unity. We see this in the general abandon- 

 ment of the old distinction between zoology and botany, 

 and the merging of the two, for all philosophical purposes, 

 in Biology, or the science of Life. We see the same in 

 the general conviction that the science of Mind is a branch 

 of the science of Life, or, at least, must be based on it. I 

 hope, in this work, to do something towards proving unity 

 of law among many of the phenomena of life and mind, 

 which at first sight may appear to have nothing in common, 

 by showing that the laws of Habit enter into all vital 

 actions whatever. But the work which has already been 



done in astronomy by Newton and in geology by Lyell 



the work, namely, of referring the most important facts of 

 those sciences to known and intelligible laws — will not be 

 done for the science of life, until the question of the origin 

 of species is as much a solved problem as the problems of 

 physical astronomy and of geology. The problems of phy- 

 sical astronomy have been solved by referring the planetary 

 motions to the ordinary laws of force. The problems of 

 geology are in process of solution by referring the facts of 



