PREFACE. 



Two things I desire to do in tiiis Preface — to indi- 

 cate briefly the aim which I have had before me in 

 the preparation of the present work, and to acknow- 

 ledge the friendly cooperation and help which have 

 contributed so Largely to give it the measure of com- 

 pleteness which it may possess. 



I must remind the reader that the work professes 

 to be a ' History ;' and as such I have endeavoured 

 to make it, so far as my opportunities would permit, 

 an exhaustive account of the organisms with which 

 it deals. I have not aimed at the terse and almost 

 epigrammatic diagnosis in which some of the early 

 systematists excelled, believing that it no longer 

 meets the requirements of science. I have sub- 

 stituted for it detailed and minute description, 

 taking account, not merely of a few salient cha- 

 racters, but of all the minor features and varietal 

 modifications of the specific type. If in some cases 

 it shonld seem that an imdue space has been allotted 



