XVI PREFACE 



for the present be prepared to admit. Nevertheless I am 

 well aware that it is but the beginning of a theory, and for 

 this reason I have presented it in the form of an inquiry 

 rather than of an established system. My plan has been 

 not so much to advance doctrines as to propound questions, 

 and to answer them with a greater or lesser degree of cer- 

 tainty, or in some cases even to leave them, to be decided by 

 future researches. I do not regard my theory as a complete 

 and perfect one, but trust that it is of such a nature as to be 

 capable of improvement and further development. 



It has been my endeavour to write as simply and intel- 

 ligibly as possible ; not as a specialist writing for specialists, 

 but as one who desires to make his case clear to all inter- 

 ested in biological problems. For this reason a number of 

 figures have been inserted, which, though perhaps super- 

 fluous for specialists, will, I trust, assist all who are less con- 

 versant with the subject, such as physiologists, medical men, 

 and indeed all interested in natural science, to a clearer 

 conception of the matters under discussion. 



As a zoologist, I have naturally, in the first instance, 

 considered the phenomena in their relation to animals, for 

 every one must base his ideas on the facts most familiar to 

 him. I have, however, done my best to lay due weight on 

 the data afforded by the study of plants, and to take into 

 account the views of botanists. It will be seen that the 

 very facts which are furnished by certain hereditary phe- 

 nomena in plants afford a strong support to certain funda- 

 mental points in my theory, and that even those which are 

 at first sight in apparent contradiction, are in reality in per- 

 fect accordance with it. 



It may perhaps be considered by medical men that I 

 ought to have brought forward more evidence with regard 



