Chap. X. 
SUMMARY. 
343 
may overlook how important a part migration must 
have played, when the formations of any one great 
region alone, as that of Europe, are considered; he may 
urge the apparent, but often falsely apparent, sudden 
coming in of whole groups of species. He may ask 
where are the remains of those infinitely numerous 
organisms which must have existed long before the 
first bed of the Silurian system was deposited: I can 
answer this latter question only hypothetically, by say¬ 
ing that as far as we can see, where our oceans now 
extend they have for an enormous period extended, and 
where our oscillating continents now stand they have 
stood ever since the Silurian epoch; but that long 
before that period, the world may have presented a 
wholly different aspect; and that the older continents, 
formed of formations older than any known to us, may 
now all be in a metamorphosed condition, or may lie 
buried under the ocean. 
Passing from these difficulties, all the other great 
leading facts in pateontology seem to me simply to 
follow on the theory of descent with modification 
through natural selection. We can thus understand 
how it is that new species come in slowly and succes¬ 
sively; how species of different classes do not neces¬ 
sarily change together, or at the same rate, or in the 
same degree; yet in the long run that all undergo 
modification to some extent. The extinction of old 
forms is the almost inevitable consequence of the pro¬ 
duction of new forms. We can understand why when 
a species has once disappeared it never reappears. 
Groups of species increase in numbers slowly, and 
endure for unequal periods of time; for the process of 
modification is necessarily slow, and depends on many 
complex contingencies. The dominant species of the 
larger dominant groups tend to leave many modified 
