9 8 SCIENCE PROGRESS. 



cell may be separated from the plant and can grow alone. 

 So that here are whole plants consisting of cells which can 

 be positively proved to have independent vitality. Now as 

 all cells grow according to the same laws, and consequently 

 the cause of growth cannot in one case lie in the cell, and 

 in another in the whole organism, and since it may be 

 further proved that some cells, which do not differ from the 

 rest in their mode of growth, are developed independently, 

 we must ascribe to all cells an independent vitality, that is 

 such combinations of molecules as occur in any single cell 

 are capable of setting free the power by which it is enabled 

 to take up fresh molecules. The cause of nutrition and 

 growth resides not in the organism itself but in its separate 

 elementary parts. . . . The manifestation of the power 

 which resides in the cell depends upon conditions to which 

 it is subject only when in connection with the whole or- 

 ganism." 



The whole theory is very succinctly summed up in the 

 following passage : " The elementary parts of all tissues are 

 formed of cells in an analogous though very diversified 

 manner, so that it may be asserted that there is one uni- 

 versal principle of development for the elementary parts of 

 organisms, however different, and that this principle is the 

 formation of cells ". 



No doubt objection may be taken to-day to the uni- 

 versality of this statement, but if we modify the last part 

 of it and read " that the most general principle of develop- 

 ment for organisms, however different, is the formation of 

 cells," we shall have very nearly expressed the truth, as we 

 know it at the present day. 



I have found it necessary to quote Schwann's work at 

 considerable length, and to repeat more emphatically what 

 I stated in my previous essay on Epigenesis and Evolution. 1 

 Dr. Whitman, 2 in a reply which deals partly with my 



1 G. C. Bourne, "Epigenesis and Evolution," " Science Progress," 

 vol. i., 1894. 



2 C O. Whitman, Evolution and Epigenesis. Boston : Ginn & Co., 

 1895. 



