Heredity and the 07'igin of Variations. 143 



processes; it undergoes fission and is reproductive. The 

 metazoa are cell-aggregates ; and in them the cells 

 exemplify a physiological division of labour. They dif- 

 ferentiate, and give rise to muscle and nerve, gut and 

 gland, blood and connective or skeletal tissue, ova and 

 sperms. Are these germinal cells mysteriously different 

 from all the other cells which have undergone differentia- 

 tion ? No. They are the cells which have been differentiated 

 and set apart for the special ivork of reproduction, as others 

 have been differentiated and set apart for other protoplasmic 

 functions. 



Cell-reproduction is, however, in the metazoa of two 

 kinds. There is the direct reproduction of differentiated 

 cells, by which muscle-cells, nerve-cells, or others reproduce 

 their kind in the growth of tissues or organs ; and there 

 is the developmental reproduction, by which the germinal 

 cells under appropriate conditions reproduce an organism 

 similar to the parent. The former is in the direct line of 

 descent from the simple reproduction of amoeba. The 

 latter is something peculiarly metazoan, and is, if one 

 may be allowed the expression, specialized in its generality. 



That the metazoa are derived from the protozoa is 

 generally believed. How they were developed is to a large 

 extent a matter of speculation. But, however originating, 

 their evolution involved the production, from cells of one 

 kind, of cells of two or more kinds, co-operating in the 

 same organism. Whenever and however this occurred, the 

 new phase of developmental reproduction must have had 

 its origin. And if in cell-division there is any continuity 

 of protoplasmic power, the faculty of producing diverse 

 co-operating cells would be transmitted. On any view of 

 the origin of the metazoa, this diverse or developmental 

 reproduction is a new protoplasmic faculty ; on any view, 

 it must have been transmitted, for otherwise the metazoa 

 would have ceased to exist. This new faculty of develop- 

 mental reproduction, then, with the inception of the metazoa, 

 takes its place among other protoplasmic faculties, and, 

 with the progress of differentiation and the division of 



