4o6 HISTORY OF THEORIES 



some definite and representative results of the modifications 

 acquired by the parental body. 



The physiological units may be compared to a band of 

 travellers who found a settlement, who build houses and arrange 

 many matters according to their " character," " tendency," 

 " individuality," " polarity " — phrase it as one will. In course 

 of time their constructed aggregate is modified by circumstances, 

 by incident forces of war, want, weather, and the like, and the 

 characters of the units are also modified ; subsequently, some 

 of them gather into " reproductive centres," which establish 

 new aggregates, largely after the likeness of the first, and yet 

 modified by the experiences endured. 



On a priori grounds, this view seems not without plausibility, 

 but Spencer's theory had to yield before the jact of germinal 

 continuity. 



Darwin's Theory of Pangenesis. — The best-known theory 

 of this class is, of course, the " provisional hypothesis of pan- 

 genesis " suggested by Darwin in his Variation of Animals and 

 Plants under Domestication (1868). The chief suggestions of 

 this theory are well known to be as follows : 



(i) Every cell of the body, not too highly differentiated, 

 throws off characteristic gemmules ; 



(2) These multiply by fission, retaining their characteristics ; 



(3) They become specially concentrated in the reproductive 



elements in both sexes ; 



(4) In development the gemmules unite with others like them- 



selves, and grow into cells like those from which they 



were originally given off, or they may remain latent 



during development even through several generations. 



We do not know whether Mr. Darwin had seriously considered 



Mr. Herbert Spencer's hypothesis of " physiological units," but, 



as Prof. Ray Lankester points out, the hypotheses might be 



called complementary. " The persistence of the same material 



gemmule and the vast increase in the number of gemmules, 



