162 GILBERT C. BOURNE. 
sitions as we may make are but resting-places for our minds 
as we ascend the mazy scale of organisation. To attempt to 
form definitions, to predicate the precise attributes of whole 
classes of phenomena, is to run counter to the very genius of 
the subject. For what do we mean by evolution if not that 
life is labile, never resting, protean in its variety? And how 
can we express this but in an incomplete way, contenting our- 
selves with particulars, and trying to show that the stream, 
though it flows in many tortuous channels, is one stream 
nevertheless. 
Cells and nuclei are protean in their variety, and since we 
very rightly insist on objective study as a preliminary to the 
understanding of them, it is not wonderful that they should 
give rise to this concept in the mind of one man, and to that 
concept in the mind of another man, and thus it is not sur- 
prising that the theory of cells should be incapable of being 
stated, as Mr. Sedgwick complains, ‘‘in so many words in a 
manner satisfactory to everyoue.” 
It is fairly obvious that Mr. Sedgwick’s quarrel with the 
cell-theory began with the dissatisfaction which he felt when 
he discovered that doctrines, which he believed to be of uni- 
versal application, were in fact contradicted by several instances. 
But he fell out of Scylla into Charybdis when he supposed 
that he could reply to a universal affirmative by a universal 
negative. 
There is an old and respectable rule of logic that of two 
contrary propositions both cannot be true and both may be 
false, whilst of two subcontrary propositions both may be 
true but both cannot be false. Had Mr. Sedgwick remem- 
bered this, he would not have attempted to overthrow the 
cell-theory by the statement of a contrary proposition of 
equally universal import. 
The cellular theory of development in the popular form in 
which it is often presented may be briefly summed up some- 
what as follows. The multicellular organism is a colony, con- 
sisting of an aggregation of separate elementary parts, viz. 
cells. The cells are independent life units, and the organism 
