374 BIOLOGY AND ITS MAKERS 
by his previous critical studies in botany. In the progress 
of the work he was impressed with the differences in ani- 
mals and the difficulty of separating one species from an- 
g va other. He had occasion to observe the variations produced 
( 
in animals through the ini influence of climate, tel temperature, 
moisture, elevation above the sea-level, etc. 
He observed also the effects of use and disuse upon the 
dev velopment of organs organs: the exercise of an organ leading to 
its greater development, and the disuse to its degeneration. 
Numerous illustrations are cited by Lamarck which serve to 
make his meaning clear. The long legs of wading birds 
are produced and extended by stretching to keep above the 
water; the long neck and bill of storks are produced by their 
habit of life; the long neck of the giraffe is due to reaching 
for foliage on trees; the web-footed birds, by spreading 
the toes when they strike the water, have stimulated the 
development of a membrane between the toes, etc. In the 
reverse direction, the loss of the power of flight in the “ wing- 
less” bird of New Zealand is due to disuse of the wings; 
while the loss of sight in the mole and in blind cave animals 
has arisen from lack of use of eyes. 
The changes produced in animal organization in this 
way were believed to be continued by direct inheritance and 
improved in succeeding generations. 
He believed also in a perfecting principle, tending to 
improve animals—a sort of conscious endeavor on the part 
of the animal playing a part in its better development. Fi- 
nally, he came to believe that the agencies indicated above 
were the factors of the evolution of life. 
His Theory of Evolution.—All that Lamarck had written 
before he changed from botany to zodlogy (1794) indicates 
his belief in the fixity of species, which was the prevailing 
notion among naturalists of the period. Then, in 1800, we 
find him apparently all at once expressing a contrary opinion, 
