ARE ACQUIRED CHARACTERS HEREDITARY? 331 
rigorous analysis. But Weismann’s greatest service lay in his con- 
structive theories rather than in destructive criticism; he forever 
disposed of theories of pangenesis and the like by showing that the 
germ cells are not built up by contributions from the body and that 
characters are not transmitted from generation to generation; but 
on the other hand that there is transmitted a germ plasm which is 
relatively independent of the body and which is relatively very stable 
in organization. This epoch-making theory of Weismann’s has natu- 
rally undergone some changes, as the result of new discoveries. 
It is no longer believed that the germ plasm is really independent of - 
the body, nor that it is absolutely stable, as Weismann at one time 
held. There is no doubt that the germ cells and the germ plasm are 
physiologically related to other cells and to other plasms, and similarly 
there is no doubt that the germ plasm although very stable can and 
does change its constitution under some rare conditions. But in the 
main the germ plasm theory is accepted by the great majority of 
biologists to-day, and recent work in genetics and cytology has brought 
many confirmations of this theory. 
Distinctions between hereditary and acquired characters.—As 
long as it was believed that the developed characters of an organism 
could be transmitted as such to its descendants it was customary to 
speak of developed characters as hereditary or acquired and to talk of 
the inheritance or non-inheritance of acquired characters. This dis- 
tinction is not a logical one for all developed characters are invariably 
the result of the responses of the germinal organization to environ- 
mental stimuli; and of course no developed character can be purely 
hereditary or purely environmental. But when a given character 
arises in many individuals of the same genotype under different 
environmental conditions it is probable that heredity, which is the 
constant factor in this case, is also the determining factor for that 
character. On the other hand if a character develops in response to 
peculiar stimuli and does not appear in other individuals of the same 
genotype in which such stimuli are lacking it is said to be an environ- 
mental or acquired character. In fine, inherited characters are those 
whose distinctive or differential causes are in the germ cells, while 
acquired characters are those whose differential causes are environ- 
mental. 
Statement of problem.—Briefly stated the question of the inheri- 
tance of acquired characters is this: Can the differential cause of a 
character be shifted from the environment to the germ plasm? Can 
