8G THE DOCTRINE OF DESCENT. 
generation. This is especially the case if one section of a 
species be in any way isolated from the rest, or if the animals 
be subjected in the course of their wanderings to novel 
conditions of life. 
The evidence from domesticated animals is very convinc- 
ing. By careful interbreeding of varieties which pleased his 
fancy or suited his purpose, man has produced numerous 
breeds of horses, cattle, sheep, and dogs, which are often 
distinguished from one another by structural differences 
more profound than those which separate two natural 
species. In great measure, however, domestic breeds are 
fertile with one another, while different species rarely are. 
The numerous and very diverse breeds of domestic pigeons, 
which are all derived from the rock-dove (Columba Jivia), 
vividly illustrate the plasticity or variability of organisms. 
It sometimes happens that the offspring of an animal 
resemble not so much the parent as some other form be- 
lieved or known to be ancestral. Thus a blue pigeon like 
the ancestral Co/umba livia may be hatched in the dovecot. 
Such reversions are not readily intelligible except on the 
theory of descent. 
(c) Historical—Among the extinct animals disentombed 
from the rocks, many form series by which those now 
existing can be linked back to simpler ancestors. Thus 
the ancient history of horses, crocodiles, and cuttle-fish is 
known with a degree of completeness which makes it almost 
certain that the simpler extinct forms were in reality the 
ancestors of those which now live. Moreover, that many 
connecting links have been discovered in the rocks, and 
that the higher animals appear gradually in successive 
periods of the earth’s history, are strong corroborations of 
the theory. 
It is less easy to state in a few words how the facts of 
geographical distribution, or the history of the diffusion of 
animals from centres where the presumed ancestral forms 
are or were most at home, favour the doctrine of descent. 
The individual life history of an animal—often strangely 
circuitous or indirect—is interpretable as a modified re- 
capitulation of the probable history of the race. 
Such, in merest outline, is the nature of the evidence 
which leads us to conclude that the various forms of life 
