LAMARCK’S THEORY OF DESCENT or 
“We are not concerned here with the shrinkage 
and shortening produced by a puckering of the parts, 
which permit ordinary extension, if instead of a con- 
tinued emptiness these viscera should be filled; the 
shrinkage and shortening in question are real, consider- 
able, and such that these organs would burst open 
rather than yield suddenly to the causes which would 
require ordinary extension. 
“In circumstances of persons of the same age, com- 
pare a man who, in order to devote himself to habitual 
study and mental work, which have rendered his di- 
gestion more difficult, has contracted the habit of 
eating lightly, with another who habitually takes a 
good deal of exercise, walks out often, and eats 
heartily ; the stomach of the first will be weakened, 
and a small quantity of food will fill it, while that of 
the second will be not only maintained in its ordinary 
health but even strengthened, 
““ We have here the case of an organ much modi- 
fied in its dimensions and in its faculties by the single 
cause of a change in habits during the life of the 
individual. 
“ The frequent use of an organ become constant by 
habit increases the faculties of this organ, even develops 
wt, and enables it to acquire dimensions and a power of 
action which it does not possess in animals which excr- 
cise less. 
“ We have just said that the lack of employment 
of an organ which necessarily exists modifies it, im- 
poverishes it, and ends by its disappearing entirely. 
“T shall now demonstrate that the continued em- 
ployment of an organ, with the efforts made to draw 
out its powers under circumstances where it would 
be of service, strengthens, extends, and enlarges this 
organ, or creates a new one which can exercise the 
necessary functions. 
“ The bird which necessity drives to the water to find 
there prey fitted for itssustenance, opens the digits of 
