396 CONCLUSION. [CHAP. XV. 



often been used by the greatest natural philosophers. The un- 

 dulatory theory of light has thus been arrived at ; and the belief 

 in the revolution of the earth on its own axis was until lately 

 supported by hardly any direct evidence. It is no valid objection 

 that science as yet throws no light on the far higher problem of 

 the essence or origin of life. Who can explain what is the essence 

 of the attraction of gravity ? No one now objects to following out 

 the results consequent on this unknown element of attraction; 

 notwithstanding that Leibnitz formerly accused Newton of intro- 

 ducing "occult qualities and miracles into philosophy." 



I see no good reason why the views given in this volume should 



\ shock the religious feelings of any one. It is satisfactory, as 



showing how transient such impressions are, to remember that the 



greatest discovery ever made by man, namely, the law of the 



attraction of gravity, was also attacked by Leibnitz, "as sub- 



/ versive of natural, and inferentially of revealed, religion." A 



celebrated author and divine has written to me that "he has 



" gradually learnt to see that it is just as noble a conception of 



"the Deity to believe that He created a few original forms 



J> " capable of self-development into other and needful forms, as to 



" believe that He required a fresh act of creation to supply the 



" voids caused by the action of His laws." 



Why, it may be asked, until recently did nearly all the most 

 eminent living naturalists and geologists disbelieve in the muta- 

 bility of species. It cannot be asserted that organic beings in a 

 state of nature are subject to no variation ; it cannot be proved 

 that the amount of variation in the course of long ages is a limited 

 quantity ; no clear distinction has been, or can be, drawn between 

 species and well-marked varieties. It cannot be maintained that 

 species when intercrossed are invariably sterile, and varieties in- 

 variably fertile ; or that sterility is a special endowment and sign 

 of creation. The belief that species were immutable productions 

 was almost unavoidable as long as the history of the world was 

 thought to be of short duration ; and now that we have acquired 

 some idea of the lapse of time, we are too apt to assume, without 

 proof, that the geological record is so perfect that it would have 

 afforded us plain evidence of the mutation of species, if they had 

 undergone mutation. 



But the chief cause of our natural unwillingness to admit that 

 one species has given birth to other and distinct species, is that 

 we are always slow in admitting great changes of which we do not 

 see the steps. The difficulty is the same as that felt by so many 

 geologists, when Lyell first insisted that long lines of inland cliffs 

 had been formed, and great valleys excavated, by the agencies 

 which we see still at work. The mind cannot possibly grasp the 

 full meaning of the term of even a million years ; it cannot add up 



