MODERN STUDIES OF EARTHQUAKES. 371 



otlier just as water waves do. Thus are explained the earthquake 

 bridges or spots which always remain unmoved through repeated 

 earthquakes, either because they are firmer, or because the progress 

 of the waves is arrested at them by interference. 



The sounds, too, which so frequently accompany earthquakes are 

 likewise simply results of this division of the waves and their escape 

 into the air, for we perceive wave motions in the air as sound. The 

 admirable delicacy of our sense of hearing is here manifested, for 

 seismic movements are not rarely perceptible, or heard, as air waves, 

 which we can not perceive as movements of the ground. Earthquake 

 thunder is caused, like storm thunder, by shocks to the air, of which 

 we hear the nearest and latest first, and the farthest and earliest last. 

 The different tone shades of the earthquake sound depend upon their 

 various sources, as from small, sharp fragments, clinking, rattling, 

 and humming; from sand and earth, dull rumbling; from trees, 

 whistling, etc. The echo in ravines not rarely operates to add 

 strength to them. Earthquake sounds that seem to come out of the 

 air from above are caused by earthquake waves reaching us by way 

 of trees, houses, etc.; the different directions and degrees of force 

 which they seem to indicate in different houses or in different rooms 

 of the same house are explainable by the different elasticity condi- 

 tions of the houses and rooms. But not the most insignificant con- 

 clusion can be drawn from these sounds concerning the nature and 

 causes of earthquakes. It is important to emphasize this fact, for 

 errors have often originated in conclusions drawn from such things. — 

 Translated for the Popular Science Monthly from the Deutsche 

 Rundschau. 



Examples of a race of curiously protectively colored mice which inhabit 

 the sandy island, the North Bull, in the Bay of Dublin, were exhibited by 

 Dr. H. Lyster Jameson in the Zoological Section of the British Association. 

 A considerable percentage of them were distinctly lighter hued than the 

 ancestral type of house mouse, though every possible gradation occurred 

 between the typical house mouse and the palest examples. The speaker 

 regarded the marked predominance of sand-colored specimens as due to 

 the action of natural selection. The hawks and owls which frequent the 

 island, and are the only enemies the "mice have to compete against, most 

 easily capture the darkest examples, or those that contrast most strongly 

 with the color of the sand. Thus a protectively colored race is becoming 

 established. The island came into existence only about a hundred years 

 ago. Consequently it is possible to fix a time limit within which the sandy- 

 colored race has been evolved. Its evolution also, as Professor Poulton 

 observed in his comment on Dr. Jameson's paper, gives additional evidence 

 to that afforded by the shore crabs described by Professor Weldon in his 

 presidential address to the section, that the transmutation of species is not 

 necessarily so slow as to be indiscernible. 



