IN -writing this volume the authors have adopted the 

 4t type-system " as being most suitable to their purpose. 

 The advantages of this system as an introduction to 

 the natural history sciences are well known, though there 

 are dangers attaching to the too exclusive restriction 

 of the attention to isolated species ; an endeavour has 

 therefore been made throughout the writing of this work 

 to keep both the advantages and the dangers in mind. 



An experience of several years in conducting correspon- 

 dence-classes in the subject has made the authors aware 

 (to a greater extent, probably, than most ordinary teachers) 

 of the wonderful way in which misunderstandings can lurk 

 in the seemingly plainest sentences, and accordingly great 

 pains have been taken to make their meaning clear at all 

 points. Only one chapter in the book (Chapter XIV.) 

 remains practically unaltered from the first edition, so that 

 while the credit for the general plan of the work belongs 

 to Mr. H. Gr. Wells, no responsibility attaches to him for 

 any part of the present book. 



The diagrams have been drawn specially for the book 

 by Mrs. Davies, and it is hoped the plan adopted for 

 labelling them will be useful to students. 



Thanks are due to Mr. J. M. Lowson, M.A., B.Sc., who 

 has read through both the manuscript and proofs, and 

 made many valuable suggestions; also to Mr. E. Stenhouse 

 for having called attention to several slips and errors. 



