464 AN INTRODUCTION TO ZOOLOGY 



Post-Darwinian Biology. 



Darwin and his contemporaries admitted that Lamarck's 

 " use inheritance " probably played some part in the formation of 

 new species, though in the main this was due to the action of 

 natural selection. Herbert Spencer upheld this view strongly, and 

 perhaps A. R. Wallace alone opposed it. 



The first naturalist to oppose it in a very thorough way was 

 August Weissmann, who, in a long series of publications starting in 

 1885, denied all transmission of what he termed " acquired cha- 

 racters." Acquired characters he defined as " those which result 

 from external influence upon the organism, in contrast to such as 

 spring from the constitution of the germ." In other words, " Modifi- 

 cations which are wrought upon the formed body, in consequence of 

 external influences, must remain limited to the organism in which 

 they arose. No such modifications of the same can be transmitted 

 to the germ cells from which the next generation springs." These 



Line of succession of 



individuals. 



Line of here- 

 ditary trans- 

 mission. 



FIG. 165. Diagram to illustrate the idea of continuity of the germ plasm. 



conclusions were reached, firstly, as a result of the exhaustive study 

 of all the instances in which such transmission was alleged to occur. 

 He was able to show that none of these cases furnished evidence of a 

 definite and incontestable character. Secondly, he made a study 

 of the mechanism whereby any characters could be transmitted. 

 As a result of these researches, Weissmann elaborated a series of 

 doctrines, known collectively as Weissmannism, the principle of 

 which is that of " the continuity of the germ plasm." According 

 to this theory, we must regard the offspring as inheriting from the 

 germ cell of the parent and not from the body of the parent. In 

 support of this, it is urged that in some animals one of the very 

 early blastomeres is definitely set aside as the producer of the germ 

 cell, and develops more or less independently of the rest. In still 

 more cases the germ mother cell is recognisable at a -very early stage, 

 as we have seen in Obelia, and, finally, in almost all the remaining 

 animals the primitive germ cells are formed before the appearance 

 of discrete orga ns. So that the body, as it were, acts as a guardian 



