5 64 THE HISTORY OF BIOLOGY 



promulgated. In a series of lectures and monographs dating from the eighties 

 he endeavours to find out what it is that produces heredity in a biological 

 sense; how, he asks, are we to account for the characteristic in organisms 

 of transmitting to their offspring their own essential being, for the fact that 

 from the eagle's egg is invariably hatched an eagle, and, moreover, one of 

 the same species as its parents? In imitation of Haeckel he starts from the 

 unicellular animals and finds that in these the mutual resemblance of the 

 different generations is due to the individuals' propagating by division; to 

 the fact that every infusorian is a segment of a previous one, that there thus 

 exists in them a " Continuitat des Individuums ." And the same is true of the 

 multicellular animals in virtue of sexual reproduction; for the individual's 

 life the sexual cells are without significance, but they preserve the continuity 

 of the species through the ages; out of them arises in certain given circum- 

 stances a new individual of the same kind as the old. 



Weismann s germinal--plasf7t theory 

 From this it may be concluded that there exists a special "germinal plasm" 

 which corresponds to the individual series in unicellular animals and which, 

 like them, preserves the species by repeated dividing, whereas the corporeal 

 plasm of the individual gradually falls into decay. Originally the differen- 

 tiation of sexual and corporeal cells had been due to a division of labour 

 in the simplest cell-colonies, such as we still see in the primitive colony- 

 forming animals; for the sexual cells that perform the function of reproduc- 

 tion contain both germinal and corporeal plasm, which separate when, in 

 the earliest embryonic stage, the rudimentary cells of the sexual organs sepa- 

 rate from the rest of the cells. Out of the germinal plasm, therefore, arises 

 the long series of analogous individuals, and these resemble one another for 

 the very reason that their form is governed by the character of the germinal 

 plasm, which is determined once and for all; if changes appear in the exter- 

 nal bodily form, they correspond to and are induced by changes in the ger- 

 minal plasm. These changes are brought about by fertilization, in which 

 the germinal plasm of two different individuals is united; through this "am- 

 phimixis," as Weismann calls it, is formed a new germinal plasm, with both 

 the parents' qualities, which accordingly appear also in the offspring. But 

 if the qualities of the individual are thus due entirely to the germinal plasm, 

 there can be no possibility of influencing the individual series from out- 

 side; the organs of the individual that are formed of corporeal plasm can 

 be influenced by practice, in so far as the germinal plasm has created possibili- 

 ties therefor, but changes of this kind exercise no influence upon the germinal 

 plasm. Consequently, Lamarck's theory that the character of the species is 

 created by the habits of the individual is untenable. 



This denial of the heredity of acquired characters became one of the 

 corner-stones of Weismann's biological theory and he sought in many and 



