PREFACE. 



The addresses and lectures contained in this volume, 

 with two exceptions, were delivered at the Marine Bio- 

 logical Laboratory during the summer session of 1890. 

 They are a continuation of the Evening Lectures begun 

 in the previous session. 



The educational value which such lectures may be pre- 

 sumed to have, and the consideration that through them 

 the aims, the needs, and the possibilities of biological 

 work might, in some measure, be made better known to 

 the public, especially to those whose liberal benefactions 

 have enabled the Laboratory to carry forward its work, 

 suggested the propriety of publication. This step, how- 

 ever, was not decided upon until late in the session of 

 this year, after most of the lectures here presented had 

 been delivered. The preparation of the Mss. for this 

 purpose has been an extra tax upon the time of the 

 contributors ; and, as this was done at my solicitation, I 

 desire to acknowledge here my great obligation to them 

 for this part of their invaluable co-operation in the work 

 of the Laboratory. 



There are one or two points in the raisojt d'etre of 

 this course of lectures, which do not lie wholly on the 

 surface, but which deserve to be made clear. It was 



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