PREFACE. XV 



lying plexus of Infusorial and Cryptogamic life, must 

 remain wholly uninfluenced, so far as their form and 

 structure is concerned, by what Mr. Darwin has 

 termed c Natural Selection.' Such vegetal and animal 

 organisms, however, gradually tend to become more and 

 more complex. An ascending development takes place, 

 and as this occurs, the causes which originally sufficed 

 to determine their form and structure, and which for a 

 time continue to induce deviations, become less and 

 less capable of bringing about structural modifications 

 during the life of the individual. Changes have now 

 to be perfected in a succession of individuals; and 

 thus is the operation initiated of those subtle and 

 more slowly modifying agencies which have been so 

 admirably illustrated by Mr. Darwin. 



Throughout this work, whilst I have been anxious 

 to consider the various aspects of the subject with as 

 much thoroughness as was necessary in order to be able 

 fairly to attempt to establish the truth of the principal 

 doctrines now advanced, I have also tried to simplify 

 the problems as much as possible. A limitation was, 

 moreover, necessitated by the pressing nature of those 

 more strictly professional duties, on account of which 

 I was first induced to enter upon these investigations, 

 and in the midst of which the work has been carried 

 on. A rich harvest, therefore, remains for many other 

 workers who may wish to develop the subject in all its 

 collateral bearings. 



These volumes being, in great part, the record of a 

 series of current investigations each section of which 

 was written whilst the next division of the subject was 



