THE HISTORY OF EVOLUTIONARY THOUGHT 



was due to the fact that evokition was "in the air" at the time. Darwin 

 himself has stated in his autobiography that he did not beheve this to be 

 true, for he had discussed his ideas with many naturaHsts oyer a period 

 of twenty years before the pubHcation of the "Origin of Species," and he 

 had not found any of them seriously inclined to agree with him. It seems 

 more probable that Kingsley was right when he said that "Darwin is con- 

 quering eyerywhere, and is rushing in like a flood by the mere force of 

 truth and fact." Darwin himself attributed his success to the fact that the 

 "Origin of Species" was highly condensed from a mass of data which had 

 been compiled and critically studied oyer a period of twenty years before 

 publication. 



Darwin realized that an understanding of heredity was essential for 

 evolutionary studies, but he apparently neyer came across Mendel's paper, 

 and he stated in the last edition of the "Origin" that the fundamental 

 principles of heredity were still unknown. To fill the need of a working 

 hypothesis, he deyised the theory of pangenesis. According to this theory, 

 all organs produce pan genes, minute particles which are carried away by 

 the bloodstream and segregated out into the gametes. Thus eyery mature 

 gamete contains a pangene from eyery organ of the animal producing it. 

 In the developing zygote, each pangene tends to cause the formation of 

 a duplicate of the organ from which it originally came. This theory plainly 

 provides for the inheritance of acquired characters. Darwin did not sug- 

 gest that this theory was correct. He proposed it simply as a working 

 hypothesis which could serve as a starting point for investigation. The 

 theory of pangenesis has been universally discarded. 



The history of evolutionary thought subsequent to the publication of 

 the "Origin of Species" may be divided, following Stebbins (personal 

 communication), into three periods: the Romantic Period, extending from 

 1860 to about 1903; the Agnostic Period, or Period of Reaction, extending 

 from 1903 to about 1935; and the Period of the Modern Synthesis, which 

 began about 1935 and is still in progress. Of course, such dates are purely 

 arbitrary: the characteristics of any period can be demonstrated in some 

 publications of earlier or later date. 



THE ROMANTIC PERIOD 



The Romantic Period was characterized by extreme enthusiasm for Dar- 

 winism, together with an uncritical acceptance of whatever data were 

 claimed to support Darwinism. Negative evidence was given little weight 

 (in contrast to Darwin's own practice), while absurd extremes of inter- 

 pretation in order to make observed facts fit Darwinian theory were quite 

 common. Leaders of this group in England included T. H. Huxley, Her- 

 bert Spencer, and George Romanes, while in the United States David 

 Starr Jordan and Asa Gray were the leaders. As a group, they went to 

 interpretive extremes, reading adaptive significance into every organic 

 structure, even on the most imaginative evidence. This was often based 

 upon excellent anatomical and taxonomic evidence, but experiments to 

 test adaptive values were unusual if not unknown. Yet it must not be 



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