JUNB 25, 1915] 



SCIENCE 



921 



prophecy as was the prediction of the exist- 

 ence of the planet Neptune. 



Similarly Weismann's assumption that 

 the determinants are arranged in a linear 

 series in the chromosomes finds strong sup- 

 port in the newest and most striking dis- 

 coveries in this field, in which Morgan is 

 able to locate at different points along the 

 length of a chromosome the determiners of 

 many developed characters. 



Finally there is at present universal 

 agreement to the declaration of Weismann 

 that no purely epigenetic theory of heredity 

 is possible, though for many years even 

 this was hotly contested. When one recalls 

 the storm of opposition which was called 

 forth by his book on "The Germ-Plasm" 

 the present acceptance, at least in principle, 

 of his major propositions can not be viewed 

 in any other light than as a triumph for 

 his theory and a tribute to the insight, fore- 

 sight and constructive ability of "Weismann. 



As a result of his theory of heredity 

 Weismann was led to investigate the gen- 

 erally accepted doctrine of the inheritance 

 of acquired characters. He carried on ex- 

 tensive 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 inher- 

 itance of acquired characters, and he chal- 

 lenged the world to furnish 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 experi- 

 ment, and for several years it appeared as 

 if no progress whatever was being made 

 toward the solution of this great question, 

 so full of importance, not merely for the 

 biologist, but also for the practical breeder 

 and indeed for the human race. But grad- 



ually there has grown up a clearer under- 

 standing 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 recog- 

 nize that inherited characters are those 

 whose distinctive or differential causes are 

 in the germ cells, while acquired characters 

 are those whose differential causes are en- 

 vironmental. No one to-day believes that 

 the developed or somatic characters of an 

 organism are transmitted to the next gen- 

 eration. To-day the problem of the inher- 

 itance of acquired characters is merely this : 

 Can changes in the environment change the 

 constitution of the germ-plasm so as to pro-, 

 duce changes in subsequent generations? 

 No one now asks whether changes in devel- 

 oped characters may be transmitted to. 

 descendants, as was generally done before 

 Weismann's work, for it is generally recog-. 

 nized that somatic characters, whether in-. 

 herited 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 en- 

 vironmental conditions, but he held that 

 such changes of the germ-plasm led to gen- 

 eral and unpredictable changes in future 

 generations which might be wholly different 

 from those somatic changes in the parents 

 which were directly produced by such en- 

 vironment. This view is now widely ac- 

 cepted. 



Thus while Weismann's views on this 

 subject underwent certain changes in the 

 course of his long life, the opinions of his 

 opponents have undergone so much greater 

 and more important changes that it may be 

 truly said that in the matter of the inher- 

 itance or non-inheritance of acquired char- 

 acters the greater portion of the scientifio 

 world has come to Weismann's position. 



