APPENDIX D. 



ON ALLEGED "SPONTANEOUS GENERATION," AND ON THE 

 HYPOTHESIS OF PHYSIOLOGICAL UNITS. 



[The following letter, originally written for publication in the 

 North American Review, but declined by 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 Prin- 

 ciples of Biology. / do this because the questions which it discusses 

 are dealt with in this volume ; and because the further explanations 

 it furnishes seem needful to prevent misapprehensions.] 



The Editor 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 Ameri- 

 can Review, 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 apparent 

 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 same two ob- 

 jections have already been made in England the one by Dr. 

 Child, of Oxford, in his Essays on Physiological Subjects, and the 

 other by a writer in the Westminster Review for July, 1 865. 



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 interpretation given by him is true. Indeed the fact that Dr. 

 Child, whose criticism is a sympathetic one, puts the same con- 

 struction on this note, proves that your reviewer has but drawn 

 what seems to be a necessary inference. Nevertheless, the infer- 

 ence 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 Phi- 

 losophy which deals with Inorganic Evolution. In the original pro- 

 gramme will be found a parenthetic reference to this omitted part, 

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



