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 



