ANIMAL EVOLUTION 15 



Spencer and also, to soni e_extant, Da j-win belie ved ; but 

 must have had some other origin, and this could only 

 be sought for in the constitution of living matter itself. 

 To show that such an origin is possible, Weismann pos- 

 tulated the existence of ultimate vital units, or biophors, 

 not very different from Spencer's physiological units, — 

 which exhibit all the attributes of life are capable of 

 multiplication, are of innumerable different kinds, and 

 owe their differences to the fact that they are themselves 

 modifiable by the action of incident forces. A simple 

 unicellular animal or plant, composed as it is of such 

 biophors, would vary in structure owing to the modifica- 

 tions induced in its component biophors, and when it 

 propagates itself by division, its characters would neces- 

 sarily be handed on to the offspring resulting from 

 division. But in a multicellular animal propagating 

 itself by sexual reproduction, the whole organism is not 

 divided, but only a minute portion is separated, and this 

 portion has the power of giving rise to the whole. This 

 minute portion is a germ-cell, and the biophors of which 

 it is composed must be sufficiently numerous, and of 

 a sufficient different number of kinds, to give rise to aU 

 the different tissues and organs of the adult. The germ- 

 cell therefore differs from the tissue cells of the adult, 

 for the latter, as a rule, can only give rise to cells of their 

 own kind, and not to the whole organism, and therefore 

 must contain only special kinds of biophors, and not all 

 the different kinds of biophors necessary for building up 

 the entire organism. 



As Darwin's theory of the circulation of gemmules is 

 rejected, for this reason among others, that it involves 

 the acceptance of the inheritance of acquired characters, 

 the observed facts of development can only be explained 

 on the supposition that the body is composed of two 

 kinds of material. The one kind, the germ-plasm, is 

 made up of all the kinds of biophors necessary to produce 

 all the different qualities of the organism ; the other 



