214 A STUDY IN HEREDITY 



some taking on one function and some another, till at length a 

 high degree of differentiation resulted, and the reproduction of 

 the race was delegated to the germ-cells. 



As I have already indicated, among unicellular organisms 

 every cell is a germ-cell, and as such is capable of continuing the 

 race. Among low unicellular organisms this power persists in 

 many cells, and the environment decides whether it shall be 

 exercised or not; thus, if almost any fragment of a sponge be 

 bedded out, it will proHferate into a complete individual. It 

 persists longer in plants than in animals ; thus from a fragment of 

 begonia leaf may arise an entire individual capable of continuing 

 the race, the cells being turned from their original destiny by a 

 change in the environment. But among higher plants this 

 power of reproducing the entire individual by means of cells other 

 than germ-cells, or what may normally proliferate into germ-cells, 

 is very exceptional. All that commonly persists is the power of 

 reproducing from such fragments of the complete organism as 

 contain cells, which might normally proliferate into germ-cells, 

 the parts wanting to render the fragments a complete organism. 

 Thus a geranium slip, for instance, contains cells which normally 

 {i.e. when the branch remains part of the plant) proliferate into 

 germ-cells. If this branch be bedded out as a slip, it produces 

 the roots which are needed to convert it into a complete organism 

 of its species. Here germ-cells are not produced from cells not 

 destined to that purpose as in the begonia leaf, but lost parts are 

 reproduced by what may be termed, and in fact is, an exaggerated 

 process of healing. In other plants the power of reproducing 

 lost parts is present in a much smaller scale, and only compara- 

 tively trifling injuries are healed ; i.e. a small fragment cannot 

 reproduce the whole, though: the whole can reproduce lost frag- 

 ments. Among animalS;, owing to the greater specialisation of 

 the cells, and the more complex condition under which they live, 

 this power of reproducing lost parts is present in general to a much 

 less extent than among plants. Low in the scale, as we see, a 

 fragment of sponge, for instance, can reproduce the whole. 

 Higher in the scale, a star-fish can reproduce a ray, a lobster a 



