THE PRESENT POSITION OF CELL-THEORY. 311 



ber, and to this extent the term is used in an equivalent 

 sense to that in which it is used in Physics. But put side 

 by side such expressions as unit of mass or unit of time with 

 the expression unit of life, and a little reflection will suffice 

 to show that the sense is inappropriate. Nor is the case 

 made better if we compare the unit of life with the chemical 

 unit. The value of the latter consists essentially in this, 

 that it is a means of dealing numerically with chemical facts, 

 and experience shows that ideas of number are very 

 appropriate to chemical facts. With life the case is very 

 different. In the present state of our knowledge the con- 

 nection between life and number is of the slenderest kind, 

 and it is insufficient to justify our applying numerical ideas 

 to vital phenomena. 



The other sense in which the term unit is used in 

 Biology is purely subjective. It stands to express our idea 

 of individuality, an idea which is founded on our own states 

 of consciousness. It is unnecessary for me to dilate upon 

 the controversies which have raged round this idea of in- 

 dividuality in its application to the animal kingdom. The 

 most acute reasoners are not agreed upon the precise 

 point where individuality ceases to belong to parts and 

 belongs to the whole even in some of the simpler colonial 

 organisms, and in such cases as the Siphonophora a satis- 

 factory solution of the problem appears to be hopeless. 



But these cases are simple in comparison with that 

 which we are now discussing. If then we cannot agree 

 about the limit of individuality in colonial organisms, how 

 are we likely to agree about the same thing in the case of 

 organic structure in general ? 



There is this to be said, however, that for us the test of 

 individuality should be a biological test, and the idea is there- 

 fore more appropriate to the question than the numerical idea 

 just spoken of. It was, no doubt, the recognition of its 

 propriety which lent such force to Schwann's argument, 

 " since it may be 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 ". 



