PREFACE TO THE SECOND EDITION.

When the First Edition of this work was published, I supposed that the general theory set forth in its Second Part, was presented in something like a finished form; but subsequent thought led me to further developments of much importance, and disclosed the fact that the component parts of the theory had been wrongly put together. Even in the absence of a more special reason, I had decided that, on the completion of the Principles of Biology, it would be proper to suspend for a few months the series I am issuing, that I might make the required re-organization. And when the time had arrived, there had arisen a more special reason, which forbade hesitation. Translations into the French and Russian languages were about to be made—had, in fact, been commenced; and had I deferred the re-organization the work would have been reproduced with all its original imperfections. This will be a sufficient explanation to those who have complained of the delay in the issue of the Principles of Psychology.

The First Part remains almost untouched: two verbal alterations only, on pp. 43 and 99, having been made to prevent misconceptions. Part II., however, is wholly transformed. Its first chapter, on “Laws in General,” is omitted, with a view to the inclusion of it in one of the latter volumes of the series. Two minor chapters disappear. Most of the rest are transposed, in groups or singly. And there are nine new chapters embodying the further developments, and serving to combine the pre-existing chapters into a changed whole. The following scheme in Edition: current; Page: [vi]which the new chapters are marked by italics, will give an idea of the transformation:—

FIRST EDITION.	SECOND EDITION.
Laws in General.	Philosophy Defined.
The Law of Evolution.	The Data of Philosophy.
The Law of Evolution (continued).
The Causes of Evolution.
Space, Time, Matter, Motion, and Force.	Space, Time, Matter, Motion, and Force.
The Indestructibility of Matter.	The Indestructibility of Matter.
The Continuity of Motion.	The Continuity of Motion.
The Persistence of Force.	The Persistence of Force.
	The Persistence of Relations among Forces.
The Correlation and Equivalence of Forces.	The Transformation and Equivalence of Forces.
The Direction of Motion.	The Direction of Motion.
The Rhythm of Motion.	The Rhythm of Motion.
	Recapitulation, Criticism, and Recommencement
The Conditions essential to Evolution.	Evolution and Dissolution.
	Simple and Compound Evolution.
	The Law of Evolution.
	The Law of Evolution.	Re-arranged with additions.
	The Law of Evolution. (continued).
	The Law of Evolution. (continued)
	The Law of Evolution (concluded).
	The Interpretation of Evolution.
The Instability of the Homogeneous.	The Instability of the Homogeneous.
The Multiplication of Effects.	The Multiplication of Effects.
Differentiation and Integration.	Segregation.
Equilibration.	Equilibration.
	Dissolution.
Summary and Conclusion.	Summary and Conclusion (Re-written).

Of course throughout this re-organized Second Part the numbers of the sections have been changed Edition: current; Page: [vii]and hence those who possess the Principles of Biology, in which many references are made to passages in First Principles, would be inconvenienced by the want of correspondence between the numbers of the sections in the original edition and in the new edition, were they without any means of identifying the sections as now numbered. The annexed list, showing which section answers to which in the two editions, will meet the requirement:—

The original stereotype plates have been used wherever it was possible; and hence the exact correspondence between the two editions in many places, even where adjacent pages are altered.

London, November, 1867.

Edition: current; Page: [viii]