VARIATION BY USE AND DISUSE. 
253 
the stems of the heather and had failed. Yet this 
heath was very extensive and very barren, and, as 
Mr. Darwin remarks, no one would ever have imag¬ 
ined that cattle would have so closely and so effectu¬ 
ally searched it for food.” 
At two different times I have observed similar 
effects from the enclosure of prairie land. As soon 
as the cattle were kept out the grass grew taller, and 
seemed to be thicker, and many flowers never before 
seen on that prairie appeared and flourished. The 
birds also became more numerous. 
Another interesting fact in connection with Mr. 
Darwin’s investigations concerns several hundred 
acres of fir-trees on the virgin heath in Staffordshire. 
By the time the fir plantation was twenty-five years 
old significant changes had taken place. There were 
many more flowers on the plantation than on the 
open heath, among them twelve new species. The 
fact that six new species of insect-eating birds appeared 
in the forest shows an increased number of insects. 
These were some of the more easily observed changes, 
but there were many others. If such have been the 
modifications of life in so small a district and in so 
short a time, what may they not be in a geological 
age, and when a whole continent is affected ? 
Mr. Clark shows that the Pampas are destitute of 
forests, not because trees are unadapted to the climate, 
as Darwin believed, but because they are destroyed 
by animal life. The dry season in that region is so 
severe that thousands of cattle perish for want of 
food and water. They eat every green thing in sight. 
