THE THEORY OF LAMARCK 37 
use, and that disuse is followed by a dwindling and 
loss of the power of action. By the inherited effects 
of use and disuse, and of modifications caused by ex- 
ternal conditions, Lamarck supposed all evolution of 
species to have come about. 
Reference has already been made to Lamarck’s de- 
scription of the method of origin of the characteristic 
form of snakes, owing to the endeavours of the snakes’ 
ancestors to creep through narrow passages. Lamarck 
was quite consistent inasmuch as he explained the 
different types which have arisen among domesticated 
species by the same theory as he applied to the origin 
of species in a state of nature. Thus he supposed the 
differences between race-horses and heavy cart-horses 
to be the direct result of the different kinds of enforced 
exercise to which the ancestors of these races were 
respectively subjected. Similarly, all the different 
breeds of dogs were supposed to have arisen owing to 
the different habits which the various successors of the 
first domesticated dogs acquired, small changes being 
accumulated by inheritance in each successive genera- 
tion. 
Turning now to species in a state of nature, the case 
of the giraffe is one of those most often quoted. 
Lamarck supposed a comparatively short - necked 
ancestor of the giraffes to have taken up the habit of 
browsing upon the leaves of trees, owing to the diffi- 
culty of obtaining other food in an arid region. In 
order to obtain their new food the animals were obliged 
to be continually stretching upward, and the effort to 
elongate their necks was attended with some small 
