200 Inheritance of Acquired Characters 
mentioned or from some other accidental cause, man 
on the contrary modifies his environment directly and 
continuously by the products of civilization. And this 
unceasing modification of the environment results in the 
unceasing evolution of the man. 
It is thus for example with cerebral development. 
Civilization itself and the continual progress of science 
and arts make steadily increasing demands upon the 
brain. And this mental exercise, steadily increasing from 
generation to generation, contributes always to the 
development of the brain. What wonder then, if the 
cranial capacity of man has become markedly increased 
even during the three last centuries, as is stated by the 
anthropologists ? 
Another cause whereby one portion of a given species 
can remain unaltered while the remaining portion 
becomes transformed, is found, when once the inherit- 
ability of acquired characters is admitted, in the sudden 
apparition of certain instincts. ‘Also in the domain of 
biology,” writes Emery, “and very especially in that 
domain, many characters of organisms seem to me to 
permit of explanation only by sudden formation. This 
is especially true of habits and instincts. How could 
the first Velleius dilatatus arrive gradually at its para- 
sitic life in the nest of the hornet? The first cuckoo 
certainly commenced suddenly to deposit its eggs in the 
nest of a strange bird.” 154 
The first sudden appearing of a new instinct can be 
compared to a happy thought. It is a definite association 
of ideas which is formed for the first time. But when 
it has once been formed, it is easily possible and indeed 
***Emery: Gedanken zur Deszendenz- und Vererbungstheorie. 
Biol., Centralbl., July 15, 1893. P. 416. 
