xviii INTRODUCTION. 



interest and keen insight — a fine example of the type no 

 longer, it is to be feared, so common in Britain as it 

 was half a century ago. 



The most important fact to be realised in regard to 

 evolution is that it is going on here and now. The crop 

 of variations which furnishes the raw materials of evolu- 

 tion is always abundant ; the directive processes of natural 

 selection and isolation are accessible to observation ; and 

 the modern methods of experimental breeding are clearing 

 up many mysteries. We are accustomed to read the 

 present in the light of the past ; but the development of 

 inductive evolution-lore is bringing nearer to us the 

 power of reading the past in the light of the present. 



Practical Importance of Birds. 



We must bring to a close these illustrations of the 

 many-sidedness of bird-study, but we cannot do so with- 

 out alluding to practical considerations. Birds play an 

 important part in the economy of nature, in preserving 

 the balance of things. It is impossible to overestimate 

 their importance in keeping injurious insects in check ; 

 without birds the earth would become uninhabitable in 

 a few years. And besides the injurious insects destroyed, 

 we have to remember the snails and slugs. Sometimes 

 careful inquiry shows that the benefit is subtler than 

 appears on the sxirface ; thus the Pied Wagtail is very 

 fond of the freshwater snails {Lymnceus truncatiikis)^ the 

 first hosts of the liver-flukes that cause the serious and 

 important disease of ' rot ^ in sheep. Many birds distri- 

 bute the seeds of useful plants and of weeds as well, and 

 those that are scattered have to be distinguished from 

 those that are digested. Darwin made a study in his 

 supremely careful fashion of what may be called ' the fauna 



