vi PREFACE. 



that the Xatui-al Sciences — ^rapid as has been their advance 

 of late in popular ftivour — are still far from occupying their 

 ftiir place in public estimation. A conviction of this would 

 probably have prevented his ever setting of his o^vn accord to 

 -vvTite for publication on the subject. But this work was 

 Avritten, in the first instance, for a different purpose, having 

 been undertaken originally in response to a call made with 

 reference to the late Meeting of the British Association at 

 Aberdeen, though, as it came to be too unmeldy for this, 

 an abstract was afterwards substituted, which was subse- 

 quently printed in the Edinburgh Philosojjhieal Journal for 

 January of last year. 



Tlie primary object of bringing forw^ard the communication 

 was to elicit discussion on views for the systematizing of one 

 of the most perplexed questions in Physiology, and the re- 

 marks to which it gave rise, though necessarily restricted, 

 went to invite their fuller examination. For this end, the 

 Author is now led to publish the Treatise itself, in all essen- 

 tial points as it originally stood, though he has submitted it 

 to a careful revision in matters of detail, and has re-written a 

 large part, for greater clearness of expression. 



At the same time, he must admit, that when his work 

 had taken shape, he felt — as in the case of a former one on 

 the Unity of Organization — that it went, in so far, whether 

 fitly or not, to occupy a void in biological literature. As the 

 nature of the subject, however, precludes its consideration, 

 except by bona fide students of Natural Science, he has made 

 no attempt, in the present work, to treat it in a popular way? 



