PREFACE TO THE REVISED EDITION. v ii 



For reasons like those named in the preface to the first 

 volume, I have not submitted the proofs of this revised 

 second volume to these gentlemen : a fact which it is need- 

 ful to name, since one or other of them might else be held 

 responsible for some error which is not his but mine. It is 

 the more requisite to say this because while, in respect of 

 matters of fact, I have, save in one or two cases, accepted 

 their corrections as not to be questioned, I have not always 

 done this in respect of matters of inference, but in sundry 

 places have adhered to my own interpretations. 



Perhaps I may be excused for expressing some satisfac- 

 tion that I have not been obliged to relinquish the views set 

 forth in 1864-7. The hypothesis of physiological units — or, 

 as I would now call them, constitutional units—has been 

 adopted by several zoologists under modified forms. So far 

 as I am aware, the alleged general law of organic symmetry 

 has not called forth any manifestations of dissent. The 

 suggested theory of vertebrate structure appears to have 

 become current ; and from the investigations of the late 

 Prof. Cope, has received verification. The conclusions 

 drawn in Part YI on " The Laws of Multiplication," have 

 not, I believe, been controverted. And though only some 

 works on botany have given currency to the doctrine set 

 forth in Appendix C, " On Circulation and the Formation 

 of Wood in Plants," yet I have met with no attempt to dis- 

 prove it. The only views contested by certain of the gen- 

 tlemen above named, are those concerning the origin of 

 the two great phsenogamic types of plants, and the origin 

 of the annulose type of animals. I have not, however, — 

 perhaps because of natural bias — found myself compelled 



