400 LAMARCK, HIS LIFE AND WORK 
vestigator, a strong advocate of the Lamarckian 
factors. In his masterly work, Studves in the Theory 
of Descent * (1875), 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. 
