x PREFACE. 



simplified the scientific facts as far as possible. 

 Thus, instead of saying that the groundsel is 

 a composite, I have said that it is a daisy by 

 family ; and instead of saying that the ascidian 

 larva belongs to the sub-kingdom Chordata, I 

 have said that it is a first cousin of the tadpole. 

 For these simplifications, I hope technical 

 biologists will pardon me. After all, if you 

 wish to be understood, it is best to speak to 

 people in words whose meanings they know. 

 Definite and accurate terminology is neces- 

 sary to express definite and accurate know- 

 ledge ; but one may use vague expressions 

 where the definite ones would convey no 

 ideas. 



I have to thank the kindness of my 

 friend the Rev. E. PURCELL, of Lincoln 

 College, Oxford, for the clever and appro- 

 priate design which appears upon the cover. 



G. A. 



