viii Preface. 



to vitalize the mass of facts which we already possess, 

 and that they may thus incite and direct new experi- 

 ments. 



If this book should serve to turn the attention of 

 others into this channel, and should thus ultimately 

 help us to a clearer insight into the nature of the forces 

 which have acted, and still act, to guide the evolution 

 of life, this result will far outweigh the acceptance or 

 rejection of the speculations which are here advanced. 



