314 
GEOLOGICAL SUCCESSION. 
Chap. X. 
These several facts accord well with my theory. I 
believe in no fixed law of development, causing all the 
inhabitants of a country to change abruptly, or simul¬ 
taneously, or to an equal degree. The process of modi¬ 
fication must be extremely slow. The variability of 
each species is quite independent of that of all others. 
Whether such variability be taken advantage of by 
natural selection, and whether the variations be accu¬ 
mulated to a greater or lesser amount, thus causing a 
greater or lesser amount of modification in the vary¬ 
ing species, depends on many complex contingencies, 
—on the variability being of a beneficial nature, on 
the power of intercrossing, on the rate of breeding, 
on the slowly changing physical conditions of the 
country, and more especially on the nature of the other 
inhabitants with which the varying species comes into 
competition. Hence it is by no means surprising that 
one species should retain the same identical form much 
longer than others; or, if changing, that it should change 
less. We see the same fact in geographical distribution; 
for instance, in the land-shells and coleopterous insects 
of Madeira having come to differ considerably from their 
nearest allies on the continent of Europe, whereas the 
marine shells and birds have remained unaltered. We 
can perhaps understand the apparently quicker rate of 
change in terrestrial and in more highly organised pro¬ 
ductions compared with marine and lower productions, 
by the more complex relations of the higher beings 
to their organic and inorganic conditions of life, as 
explained in a former chapter. When many of the 
inhabitants of a country have become modified and im¬ 
proved, we can understand, on the principle of competi¬ 
tion, and on that of the many all-important relations 
of organism to organism, that any form which does 
not become in some degree modified and improved. 
