1883.] Recent Literature. 1043 
e., whether there exists a special “ developmental force.” This 
he believes cannot be decided by mere’ speculation, “ it must also 
be attempted to approach it by the inductive method.” While 
Weismann makes use of Darwin’s principle of natural selection, 
he also accepts “ the transforming influence of direct action, as 
upheld by Lamarck,” although he adds, “its extent cannot as 
yet be estimated with any certainty.” 
The work consists of five essays, The first is on the “seasonal 
dimorphism of butterflies,” in which the author attempts to dis- 
cover the causes of this remarkable dimorphism, and by this 
. means to indicate at the same time the extent of one of the trans- 
forming factors with reference to a definite case. How stimu- 
lating the facts and results given in this chapter are to entomolo- 
gists, whether they are interested in the philosophic bearings of 
the fact or not, is well known; new and suggestive lines of re- 
search have been opened by the author, and in this country car- 
ried on by Mr. W. H. Edwards. 
Weismann concludes from his studies in seasonal dimorphism, 
“that differences of specific value can originate through the 
direct action of external conditions of life on y.” He has cer- 
tainly proved that new species arise by differences in climate, 
while he also (in a note*to the English edition) concedes that 
sexual selection plays a very important part in the markings an 
coloring of butterflies, but he significantly adds, “ that a change 
Produced directly by climate may be still further increased by 
sexual selection.” 
A second point, and one of particular interest, which the 
author claims to be elucidated by seasonal dimorphism, is “ the 
origin of variability.” Having shown that “secondary forms 
are for the most part considerably more variable than _pri- 
Mary forms,” it follows that “ similar external influences either 
induce different changes in the different individuals of a species, 
or else change all individuals in the same manner, variability 
arising only from the unequal time in which the individuals are 
€xposed to the external influence. The latter is undoubtedly the 
causes in the same manner, or better, in the same direction.” 
He then concludes : 
fixed direction, determined by the physical constitution of the 
Species. When, however, new climatic forms of butterflies, in 
