PREFACE. 



SEVERAL months ago I published in the 

 Fortnightly Review a lecture, which I had pre- 

 viously delivered at the Philosophical Institutions 

 of Edinburgh and Birmingham, and which bore 

 the above title. The late Mr. Darwin thought 

 well of the epitome of his doctrine which the 

 lecture presented, and urged me so strongly 

 to republish it in a form which might admit of 

 its being "spread broadcast over the land," 

 that I promised him to do so. In fulfilment 

 of this promise, therefore — which I now regard 

 as more binding than ever — I reproduce the 



