\The following letter, originally W7-itten for publication in the North 

 American Review, but declined hj the Editor in pursuance of 

 a general rule, and eventually otherwise published in the United 

 States, I have thought well to append to this first volume of the 

 Principles of Biology. / do this because the questions lohich it 

 discusses are dealt with in this volume ; and because the further ex- 

 planations it furnislies seem needful to prevent misapprehensions^] 



ON ALLEGED " SPONTANEOUS GENERATION," AND ON TPIE 

 HYPOTHESIS OF PHYSIOLOGICAL' UNITS. 



The Editm' of the North American Review. 

 Sir, 



It is in most cases unwise to notice adverse criticisms. 

 Either they do not admit of answers or the answers may be left to 

 the penetration of readers. When, however, a critic's allegations 

 touch the fundamental propositions of a book, and especially when they 

 appear in a periodical having the position of the North American 

 lieview, the case is altered. For these reasons the article on 

 " Philosophical Biology," published in your last number, demands 

 from me an attention which ordinary criticisms do not. 



It is the more needful for me to notice it, because its two leading 

 objections have the one an actual fairness and the other an apparant 

 fairness ; and in the absence of explanations from me, they will be 

 considered as substantiated even by many, or perhaps most, of those 

 who have read the work itself — much more by those who have not 

 read it. That to prevent the spread of misapprehensions I ought to 

 say something, is further shown by the fact that the sametvro objec- 

 tions have already been made in England — the one by Dr. Child, 

 of Oxford, in his Essays on Pliysiological Subjects, and the other by 

 a writer in the Westminster Review for July, 1865. 



In the note to which your reviewer refers, I have, as he says, tacitly 

 repudiated the belief in "spontaneous generation;" and that I have 

 done this in such a way as to leave open the door for the interpre- 

 tation given by him is trne. Indeed the fact that Dr. Child, whose 

 criticism is a sympathetic one, puts the same construction on this 

 note, proves that your reviewer has but drawn what seems to be a 

 necessary inference. Nevertheless, the inference is one which I did 

 not intend to be drawn. 



In explanation, let me at the outset remark that I am placed at a 

 disadvantage in having had to omit that part of the System of 

 Philosophy which deals with Inorganic Evolution. In the orginal 

 programme will be found a parenthetic reference to this omitted 

 part, which should, as there stated, precede the Principles of Biology, 



