192 FOOT-NOTES TO EVOLUTION. 
As Professor Bergen has well said, “It is important 
that we should understand that none of the kinds of 
evidence in favour of evolution loses so 
much by being represented only by 
scattered instances as the argument from 
distribution.” And, conversely, no argument is more 
conclusive when all the known facts are brought into 
consideration together. The universal fact of the muta- 
bility of species can be really understood or appreciated 
only by him who has seen with his own eyes the changes 
in multitudes of species. To the ordinary observer the 
species seem constant, just as the face of a cliff seems 
constant. To the student of Nature, mutability is 
everywhere. Just as the wind and rain and frost quietly 
but surely change the face of a cliff, so do other forces 
of Nature as quietly but as surely change the face of a 
species. 
It was this phase of the subject, the relation of spe- 
cies to geography, which first attracted the attention 
both of Darwin and Wallace. Both these observers 
noticed that island life is neither strictly like nor unlike 
the life of the nearest land, and that the degree of 
difference differs with the degree of isolation. Both 
were led from this fact to the theory of derivation, and 
to lay the greatest stress on the progressive modifica- 
tion resulting from the struggle for existence. 
In the voyage of the Beagle Darwin was brought 
in contact with the singular fauna of the Galapagos 
Islands, that cluster of volcanic rocks 
which lies in the open sea about six hun- 
dred miles west of the coasts of Ecuador 
and Peru. Thesea birds of these islands are essentially 
the same as those of the coast of Peru. So with most 
of the fishes. We can see how this might well be, for 
both sea birds and fishes can readily pass from the 
Cumulative 
evidence. 
The fauna of the 
Galapagos. 
