Embryology. 109 
another ; but, as a rule, they continue to be held in more 
or less close apposition by means of other cells and 
binding membranes,—with the result of giving rise to 
those various “ tissues,” which in turn go to constitute 
the material of “organs.” I cannot suppose, however, 
that any advocate of discontinuity will care to take 
his stand at this point. But, if any one were so 
foolish as to do so, it would be easy to dislodge him 
by describing the state of matters in some of the 
Protozoa where a number of unicellular “individuals ”’ 
are organically united so as to form a “colony.” 
These cases serve to bridge this distinction between 
Protozoa and Metazoa, of which therefore we may 
now take leave. 
In the second place, there is the no less obvious 
distinction that the result of cell-division in the 
Metazoa is not merely to multiply cells all of the 
same kind: on the contrary, the process here gives 
rise to as many different kinds of cells as there are 
different kinds of tissue composing the adult organism. 
But no one, I should think, is likely to oppose the 
doctrine of continuity on the ground of this distinc- 
tion. For the distinction is clearly one which must 
necessarily arise, if the doctrine of continuity between 
unicellular and multicellular organisms be true. In 
other words, it is a distinction which the theory of 
evolution itself must necessarily pre-suppose, and 
therefore it is no objection to the theory that its 
pre-supposition is realized. Moreover, as we shall 
see better presently, there is no difficulty in under- 
standing why this distinction should have arisen, so 
soon as it became necessary (or desirable) that indi- 
vidual cells, when composing a “colony,” should 
