viii PREFACE. 



or cause disease as are necessary to an intelligent 

 understanding of the subject, and of the kind 

 of treatment called for. Consequently I have 

 avoided any unnecessary classification or elaborate 

 descriptions of parasitic fungi or insects, histo- 

 logical details of the tissues of plants, chemical 

 and physical details regarding the soil, and even 

 matters purely physiological as far as possible. 

 Several admirable works on these subjects are 

 already available, and must be referred to for 

 further details. 



It is, however, quite out of the question to 

 avoid technicalities, though I have chosen the 

 simpler course wherever it was found feasible, 

 and have tried to so employ the examples selected 

 that the student who wishes to go further into 

 the matters dealt with may turn to special 

 treatises for further information. For one emi- 

 nently technical section I ought perhaps to 

 apologise, but the temptation to try and set 

 forth, in concrete form and suitable for the pur- 

 poses of this book, some account of what is known 

 of the most essential and profound factors con- 

 cerned in the difficult question of the nature of 

 life and death, health and disease, was great. 

 Probably my apology should go further, and 

 apply to what after all must be failure to explore 

 this mystery to the bottom : my only excuse 

 must be that it may stimulate others to go 

 further. 



It was an afterthought to add, in Part I., the 

 considerations on the factors which influence the 



