238 SCIENCE PROGRESS. 



The conclusion is this : that life is possible only when two 

 (or more) substances of complex chemical constitution are 

 brought together, and that when these two (or more) substances 

 ai'e brought together we have before us a cell. The cell there- 

 fore is the vital unit /car' e^o^V. The component parts of 

 the cell are not vital units, for by themselves they are in- 

 capable of life; they are the auxiliaries, the indispensable 

 auxiliaries of life, but they are not themselves living. 



This is not a theory of life, and it does not pretend to 

 be one. It is the generalisation which the facts seem to 

 warrant, and if it be true, as I believe it must be true, it is 

 entirely inconsistent with the whole group of theories based 

 upon hypothetical biophors, gemmules, plasomes, physio- 

 logical units, plastidules et hoc genus omne. Those theories 

 are false. And the cell theory is not inadequate, but it is 

 the only theory which our knowledge of structure and of 

 life processes permits us to adopt, at least if we confine 

 ourselves to that part of it which is essential, namely, that 

 there is one general principle for the formation all tissues, 

 animal and vegetable, and that principle is the formation of 

 cells. 



Cells are the ultimate vital units, though they are not 

 the ultimate structural units ; they are the Lebenstrager, or 

 biophors, and there are no living individuals lower than 

 cells. 



As I have made an effort to stick to facts and have 

 slighted hypotheses, I shall doubtless incur the profound 

 contempt of those superior persons who find no mental 

 repose in things which can be clearly apprehended, but 

 must leave the material support of earth and seek for rest on 

 the unsubstantial pillows of cloudland. They will have 

 abundant scope for exercising their contempt, for my con- 

 clusion explains nothing, and gives no clue to the problems 

 of heredity. 



As I have said in the earlier part of this essay, I have 

 no intention to discuss here the complicated problems which 

 are involved in the question of heredity. I take my stand 

 on the position from which I started, namely, that if minute 



