950 



SCIENCE. 



[N. S. Vol. XXIII. No. 599. 



to be considered as well known, being repre- 

 sented by indisputable facts. 



Inheritance is a fact which can not be de- 

 nied, but the causes of inheritance are un- 

 known. However, we possess theories with 

 regard to it, one of which is Weismann's 

 germ-plasm theory. I am not going to dis- 

 cuss this here. The latest investigations on 

 the minute processes in fertilization, as well 

 as experiments on heredity, go far to advance 

 our knowledge as to the causes of inheritance, 

 but at present it is impossible to say to what 

 end they finally may lead. 



Variation is antagonistic to inheritance, and 

 is also a fact. For a long time its cause 

 seemed to be plain, and Darwin held the opin- 

 ion that it is due to changes of environment, 

 and he believed at the same time that changes 

 thus produced might become hereditary 

 (' Origin of Species,' in the very beginning 

 of chapter 1, p. 5; further on, p. 8, and then 

 again in chapter 5, p. 103). In this respect 

 Darwin was entirely upon the standpoint of 

 Lamarck, who was the first to express the idea 

 of evolution in consequence of inheritance of 

 acquired characters, chiefly by use and non- 

 use (here we have the recognition of two 

 principles : inheritance and variation) . Later, 

 a different opinion began to prevail, namely, 

 that acquired characters, such as are due to 

 external stimuli, are not transmitted, and that 

 only variations of another class, which have a 

 different cause, are inherited. These are the 

 so-called ' spontaneous,' ' germinal ' or ' con- 

 genital ' variations. This view was chiefly 

 defended by Weismann, although he was not 

 the first to propose it. Finally, de Vries main- ■ 

 tains that it is mutation, and not variation 

 that is inherited," or more correctly that it is 

 only a certain form of variation that is trans- 

 missible (connected with the species-making 

 process), namely, that which is represented by 

 sudden leaps. 



Thus we see that the main dispute was with 

 reference to the causes of variation, and we 

 can distinguish three chief theories, which do 



° This is all that remains of de Vries's views 

 after they have been stripped of their most obvious 

 fallacies. 



not entirely correspond to the scheme given 

 by Vaughan. 



1. Dynamic theory (Dall). Evolution is 

 started by variation due to external stimuli; 

 these variations are transmissible to the off- 

 spring. 



In this general view, we have to distinguish 

 a development in four steps, each representing 

 an improvement upon the older ideas, but not 

 being contrary to them. 



(ffl) Lamarckian view : two factors are recog- 

 nized — ^variation and inheritance. Variations 

 are called adaptations. 



(b) Darwinian view: three factors are recog- 

 nized — variation, inheritance and natural se- 

 lection (struggle for life). Variations are not 

 always adaptations, but may be disadvanta- 

 geous. The struggle for life disposes of them. 

 \ fourth factor (speciation) is also indicated 

 by Darwin, but not clearly recognized. 



(c) Wagnerian view: addition of the fourth 

 factor segregation (separation) as producing 

 speciation. 



{d) Pfeffer's correction of Darwin's concep- 

 tion of natural selection. 



2. The view that not all variations are 

 caused by external stimuli, and that not all 

 variations are transmissible, but only those 

 that are due to ' inner ' causes. This view 

 was held formerly by Weismann, but is now 

 abandoned by him practically, although not 

 professedly. This view is at present often 

 called the Darwinian hypothesis, but wrongly 

 so. 



3. The view of de Vries. He also contends 

 that only a certain class of variations is trans- 

 missible, that is to say, may start the forma- 

 tion of new species. This class is what he 

 calls mutations. As to the causes of muta- 

 tion de Vries is noncommittal. 



I, for my part, accept the dynamic theory 

 with all its amendments. I decline to con- 

 sider the two other views, the third for reasons 

 set forth recently." I have also given my 

 reasons for rejecting Weismann's views," but 

 it might be well to condense here again, why 

 I believe that the theory of the transmission 

 of acquired characters possesses a better title 



'"Science, May 11, 1906. 



"■Biolog. Centralblatt, 18, 1898, p. 139 flf. 



