PREFACE TO THE FOURTH EDITION. XV 



ence to the influence of the state of "expectant attention" on the ope- 

 rations of Nutrition, Secretion, &c. 



The additions and alterations which have been made in Chapter xix. 

 " On Generation," will be found to be both numerous and important, 

 especially under the Section on the "Development of the Embryo;" 

 which has been almost entirely rewritten, so .as to bring the view of 

 this process more into accordance with the existing state of our know- 

 ledge of it. The Author has not felt it expedient, however (for the 

 reasons mentioned in 922), to enter into minute details upon this sub- 

 ject.. 



In Chapter xx., " On the Different Branches of the Human Family, 

 and their Mutual Relations," all that directly relates to its subject has 

 been considerably extended, and many novelties have been introduced ; 

 whilst those arguments for the Specific Unity of the Human Races, 

 which are derived from the analogy of the lower animals, have been 

 simply referred to, having been fully dwelt on by the Author else- 

 where. 



The closing Chapter, " On Death," has been almost entirely written 

 for this Edition ; the subject having been only touched on incidentally 

 in the preceding. 



The Author trusts that it will be apparent, from the foregoing sum- 

 mary, that he has spared no pains to render the present Edition worthy 

 of the favorable reception which has been accorded to its predecessors. 

 The principle he has adopted throughout, has been that of making the 

 Treatise express his present convictions and opinions, as completely as 

 if it had now been for the first time put forth ; the old materials having 

 been incorporated with the new, rather than the new with the old ; and 

 having only been employed, where they could be readily made subser- 

 vient to this purpose. In making his selection from the vast mass of 

 results which have been recently accumulated by the diligent labors of 

 Physiologists of various countries, the Author has been guided by the 

 principle which he expressed in the preface to his previous Edition ; 

 that, namely, of not rashly introducing changes inconsistent with usually 

 received views ; nor, on the other hand, showing an unwillingness to 

 reject the statements of those who have taken adequate pains to arrive 

 at accurate conclusions. " He trusts that he may be found" now as 

 then " to have exercised a sound discretion, both as to what he has 

 admitted, and what he has rejected ; and that his work will appear to 

 exhibit, on the whole, a faithful reflection of the present aspect of Phy- 

 siological Science. He cannot venture to expect, however, that he has 

 succeeded in every instance, so that each of his readers will be in con- 



