400 LAMARCK, HIS LIFE AND WORK 



vestigator, a strong advocate of the Lamarckian 

 factors. In his masterly work, Studies in the Theory 

 of Descent* {12,7s), although accepting Darwin's prin- 

 ciple of natural selection, he also relied on " the 

 transforming influence of direct action as upheld by- 

 Lamarck," although he adds, " its extent cannot as 

 yet be estimated with any certainty." He con- 

 cluded from his studies in seasonal dimorphism, 

 " that differences of specific value can originate 

 through the direct action of external conditions of 

 life only." While conceding that sexual selection 

 plays a very important part in the markings and 

 coloring of butterflies, he adds " that a change pro- 

 duced directly by climate may be still further in- 

 creased by sexual selection." He also inquired into 

 the origin of variability, and held that it can be 

 elucidated by seasonal dimorphism. He thus formu- 

 lated the chief results of his investigations: "A 

 species is only caused to change through the influ- 

 ence of changing external conditions of life, this 

 change being in a fixed direction which entirely de- 

 pends on the physical nature of the varying organ- 

 ism, and is different in different species or even in 

 the two sexes of the same species." 



The influence of changes of climate on variation 

 has been studied to especial advantage in North 

 America, owing to its great extent, and to the fact 

 that its territory ranges from the polar to the tropi- 

 cal regions, and from the Atlantic to the Pacific 



* Studies in the Theory of Descent. By Dr. August Weismann. 

 Translated and edited, with notes, by Raphael Meldola. London, 

 1882. 2 vols. 



