166 OUTLINES OF EVOLUTIONARY BIOLOGY 



and behaviour of the " gemmules " that it cannot be accepted as 

 more than what Darwin himself termed it, a provisional 

 hypothesis or speculation. 



It is interesting to observe that Darwin finds the cause of 

 variation in the direct influence of the environment, including 

 under that term the effects of use and disuse upon the organism. 

 In this respect he agrees with the views of Lamarck and differs 

 widely from those of Weismann and many other modern 

 biologists, who deny, either totally or in part, the possibility of 

 the inheritance of acquired or somatogenic characters. It 

 will have been noticed that the theory of pangenesis is of an 

 essentially pre-formationist character, for it assumes the existence, 

 within the fertilized egg, of an immense number of material 

 particles (gemmules) which in some way or other represent the 

 different inheritable characters of the body. 



The celebrated theory of heredity which we owe to Professor 

 Weismann ' is based upon what he terms the " Continuity of 

 the Germ Plasm." The general idea of continuity is, of course, 

 by no means a new one ; indeed the protoplasmic continuity of 

 parent and offspring, through the germ cells, must form the 

 material basis for the transmission of characters on any theory 

 of heredity. Weismann, however, gives much greater precision 

 to the idea than any of his predecessors. He identifies the 

 chromatin of the nucleus as the actual hereditary substance, the 

 bearer of all inherited tendencies, and draws a very sharp dis- 

 tinction between the somatic cells which, with almost endless 

 diversity of form .and function, build up the body of one of the 

 higher plants or animals, and the germ cells, which play little if 

 any part in the life of the individual in which they are lodged 

 but are destined, under favourable circumstances, to give rise to 

 the next generation. 



We have already seen that the germ cells are frequently 

 separated from the somatic cells at a very early stage in develop- 

 ment. It may be that the distinction between the two is actually 

 inaugurated by the very first division of the fertilized ovum, as 

 in the horse worm, Ascari? meyalocepliala (p. 129), or it may be 

 recognizable at the gastrula stage, as in the arrow worm, Sagitta 



1 For full accounts of this theory see the English translations of Weismann's two 

 chief works on the subject, "The Germ Plasm " (Contemporary Science Series, 

 1893) and "The Evolution Theory" (Edward Arnold, 1!)04). 



