APPENDIX D 213 



are, as is well-known, exceptions to the above ; unending repro- 

 ductions may occur without conjugation, as among such plants as are 

 propagated by slips or suckers, and self-fertilisation also occurs, but 

 the general rule is as I have stated. A multicellular plant or 

 animal in the successive stages of its development is, therefore, the 

 homologue, not of the remote ancestral unicellular organism, but 

 of all those successive generations of unicellular organisms which 

 intervene between one act of conjugation and the next. 



Unlike the cell descendants of a conjugated unicellular 

 organism, the cell descendants of a conjugated germ differ from 

 it, and from one another, in that they undergo differentiation 

 along certain definite Hnes (into nerve, muscle, bone, etc.), the 

 germ-cells being so specialised that the cell-communities which 

 spring from them are very like the cell-community of which they 

 were cell-members ; for which reason a man, for instance, is like 

 his parent. Moreover, the cell-descendants of a conjugated 

 germ differ from the cell-descendants of a conjugated unicellular 

 organism in that they remain adherent, and in that, in different 

 lines of descent, they multiply at different though definite rates. 

 Did the cell-descendants of the germ all multiply at an equal rate, 

 a solid spherical mass of cells would, of course, result ; whereas, 

 owing to differences in their rates of multiplication, the shape 

 of multicellular plants and animals are irregular (i.e. not spherical). 

 But, though these rates of multiplication in different lines of 

 descent are pretty definite in every species of plant and animal, 

 they differ widely in different species, whence arise differences in 

 shape betwixt one species and another. An ox, for instance, 

 differs in shape from a man because in it the cells, in different 

 lines of descent, do not multiply at the same rate as in the man. 



We cannot doubt that, when first multicellular organisms 

 were evolved from unicellular, all the cells constituting the mass 

 were morphologically and physiologically similar, and that, there- 

 fore, like the ancestral unicellular organism, every cell was capable 

 of performing all the functions of life — food-getting, locomotion, 

 reproduction of race, etc. Later, as a result of Natural Selection, 

 differentiation appeared among the adherent cells of the community, 



