REPRODUCTION 61 



man, differentiation is carried to a very great 

 distance. Yet even in man the cells of the brain, 

 of the liver, of bone, muscle and skin, so different 

 from one another, so utterly unconvertible into one 

 another, are all the descendants of a single cell 

 and have been derived from it by processes similar 

 to those described in the earlier part of the present 

 chapter. Perhaps one may pause for a moment 

 here to consider the bearing of these facts upon 

 the question with which this book is concerned. f < J 

 The living body is often compared to a machine Ljving body 

 and is said by some to be nothing but a machine *^ d machme 

 and explicable, did we know all the facts, on the 

 same laws and principles as those whereby a ^ 

 machine is explained. Those who hold such views / 

 have, at least so it seems to the present writer, to 

 encounter enormous, even insuperable, difficulties, 

 when they arrive at this subject of development ; 

 for let us grant that the cell the single cell is a 

 machine for the purposes of argument. Let us 

 even suppose that such a machine should be cap- 

 able of producing, of its own mere motion, other 

 machines like unto itself. That is a sufficiently 

 large assumption, since no machme has ever yet A 

 been made or thought of which does anything even i 

 faintly foreshadowing what is here imagined. Nor V 

 has any chemical compouncL the power of redupli^ 

 eating itself, by means of its own inherent forces. 



