316 THE POPULAR SCIENCE MONTHLY. 



Still, again, the force of the argument drawn from embryology 

 does not come from a knowledge of the changes in a single egg. 

 All these studies need the second premise, obtained by years of 

 comparison in different fields of investigation, that no case is iso- 

 lated. Without this premise, the argument would be incomplete. 

 The few cases of development or change which can be brought to 

 popular notice are simply illustrations and not proofs. 



As Prof. Bergen has well said, "it is important that we 

 should understand that none of the kinds of evidence in favor of 

 evolution loses so much by being represented only by scattered 

 instances as the argument from distribution." And, conversely, 

 no argument is so strong when all the known facts are brought 

 into consideration together. The universal fact of the mutability 

 of species can be really understood or appreciated only by him by 

 whose eyes multitudes of species have been seen to change. To 

 the ordinary observer the species seems 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. 



And now we may notice that it was precisely this phase of the 

 subject, the relation of species to geography, which first attracted 

 the attention of both Mr. Darwin and Mr. 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 varies 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 modification resulting from the struggle 

 for existence. 



In the voyage of the Beagle, you remember, Mr. Darwin was 

 brought in contact with the singular fauna of the Galapagos Isl- 

 ands, that cluster of volcanic rocks which lies in the open sea 

 some six hundred miles west of the coast of Equador and Peru. 

 The sea 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 one region to the other. But the land birds, as well as 

 the reptiles, insects, and plants, are mostly peculiar to the islands. 

 The same species are found nowhere else. But other species very 

 much like them in all respects are found, and these all live along 

 the coast of Peru. In the Galapagos Islands, according to Dar- 

 win's notes, " there are twenty-six land birds ; of these, twenty-one 

 or perhaps twenty-three are ranked as distinct species, and would 

 commonly be assumed to have been here created ; yet the close 

 affinity of most of these birds to American species is manifest in 

 every character, in their habits, gestures, and tones of voice. So 



