viii THE STUDY OF EVOLUTION 



us with reference to matters that form the basis of general 

 biology. Hence the Croonian Lectures upon Adaptation and 

 Disease, delivered in June 1917, which form the first part of 

 this volume. 



Elsewhere I have dealt with this subject of adaptation for 

 the benefit of the student of medicine ; x in dealing with the 

 same subject more from the point of view of the biologist,. I 

 had of necessity to refer to and repeat data and deductions 

 employed in my earlier writings. The views here enunciated 

 have been arrived at in orderly sequence from the year 1891 

 onwards, and it has been difficult to review that sequence with- 

 out employing the facts and arguments by which the successive 

 steps were attained. 



While making this confession I have at the same time found 

 myself confronted with the difficulty that, in compressing my 

 treatment of the subject into four lectures, much that should 

 have been dealt with in fuller detail, and with greater wealth 

 of examples, has owing to the exigencies of space and time 

 been unavoidably passed over very rapidly. To remedy this, 

 and at the same time to demonstrate that no newly developed 

 doctrine is here taught, but one arrived at and published many 

 years ago, it has seemed advisable to reprint in their entirety a 

 certain earlier papers and addresses, all bearing upon adapta- 

 tion and tissue modification, which establish and amplify the 

 successive stages of my argument. These are useful as documents 

 in the case. Personally, re-reading them after a lapse of many 

 years, and so with some sense of detachment, they have struck 

 me as to a certain extent entertaining. May their other readers 

 have the same opinion, and obtain a like satisfaction ! 



It may be held that an apology is due to not a few well-known 



1 Notably in the two editions of my Principles of Pathology in 1908 and 

 1910, but those are works of some thousands of pages ; it is perhaps natural 

 that biologists in general have been deterred from consulting them. 



2 Save for slight structural alterations, advisable in converting delivered 

 addresses from the spoken to the printed form. These alterations have been 

 duly noted whenever it has seemed possible that there has been introduced 

 thereby the slightest departure from the sense of the original. 



