AUGUST WEISMANN. ix 



in principle, of his major propositions cannot be viewed in any other 

 light than as a triumph for his theory and a tribute to the insight, 

 foresight and constructive ability of Weismann. 



As a result of his theory of heredity Weismann was led to in- 

 vestigate the generally accepted doctrine of the inheritance of ac- 

 quired characters. He carried on extensive experiments in order 

 to learn whether mutilations of parents through many generations 

 were ever inherited by offspring; he investigated many supposed 

 cases of the inheritance of such characters, and as a result of this 

 work he was led to deny altogether the possibility of the inheri- 

 tance of acquired characters, and he challenged the world to fur- 

 nish any satisfactory proof of such inheritance. This work of 

 Weismann's called forth a tremendous amount of discussion and a 

 relatively small amount of direct observation and experiment, and 

 for several years it appeared as if no progress whatever was being 

 made toward the solution of this great question, so full of im- 

 portance, not merely for the biologist but also for the practical 

 breeder and indeed for the human race. But gradually there has 

 grown up a clearer understanding of the problem and of what is 

 meant by " inherited " and " acquired " characters, and gradually 

 this dead-lock of opinions is breaking up. Now we recognize that 

 inherited characters are those whose distinctive or differential causes 

 are in the germ cells, while acquired characters are those whose 

 differential causes are environmental. No one today believes that 

 the developed or somatic characters of an organism are transmitted 

 to the next generation. Today the problem of the inheritance of 

 acquired characters is merely this : Can changes in the environment 

 change the constitution of the germ-plasm so as to produce changes 

 in subsequent generations? No one now asks whether changes 

 in developed characters may be transmitted to descendants, as was 

 generally done before Weismann's work, for it is generally recog- 

 nized that somatic characters whether inherited or acquired are 

 not transmitted from generation to generation, the only thing which 

 is transmitted being the germ-plasm. Weismann admitted in his 

 later writings that the germ-plasm might be modified to a limited 

 extent by certain environmental conditions, but he held that such 



