TJNiOBSITY 



PREFACE. 



WHEN this Essay was first published, the following was the 

 prefatory note (October 1869) : 



" The substance of the greater part of this paper, which has 

 been in the present form for some time, was delivered, as a 

 lecture, at a Conversazione of the Royal College of Physicians 

 of Edinburgh, in the Hallof the College, on the evening of Friday, 

 the 30th of April last. 



It will be found to support itself, so far as the facts are con- 

 cerned, on the most recent German physiological literature, as 

 represented by Rindfleisch, Kiihne, and especially Strieker, with 

 which last, for the production of his * Handbuch,' there is as- 

 sociated every great histological name in Germany." 



I may now state, without any more particular reference to 

 the motives, whether general or special, which gave rise to it, 

 that this essay of mine had but one thing to do. to protest, 

 namely, against the thoughtless extinction of certain essential 

 differences in a supposed common identity. I may illustrate this 

 by a remark in a letter to me on the subject by the late lamented 

 Professor Ueberweg, which (the letter itself is dated Jan. 16, 

 1870) is as follows : " As I am neither a physiologist nor a 

 zoologist, I cannot be expected to follow your argument 

 into its details, but I am vividly interested by its logical 

 or dialectical leading thought the contention, namely, for the 

 right of the logical category of Difference, as against that of 

 Identity one-sidedly accentuated, as it seems, by Huxley." My 

 reply to this was, " that he (Ueberweg) had hit the mark that 

 I had been simply laughing all through, and holding up to the 

 category of identity, the equally authentic category of difference 

 but that it had taken a German to find me out." 



In the same letter, Ueberweg proceeded to say that the 

 question, in the first place, therefore, was evidently a logical one. 

 Now this, doubtless, is true ; but this, nevertheless, is not 



