284 POPULAR SCIENCE MONTHLY. 



THE EVIDENCE OF SNAILS ON CHANGES OF LAND 



AND SEA. 



By henry a. PILSBRY, Sc.D., 

 philadelphia academy of natural sciences. 



TF we wish to learn the history of any land area, we turn to its 

 -■- geology for a record of changes in the past. The time of its 

 emergence from ocean, the age of its mountains and the details of its 

 growth by successive increments of land elevated from the sea, all this 

 we may expect to learn with reasonable accuracy, besides gaining a 

 knowledge of the plants and animals which lived from time to time 

 upon the coasts. 



But we may push our inquiry beyond the shore, and ask. Over 

 this expanse of sea did land once extend ? Did an arm of the land reach 

 to this island in the old time, or are the islands of that archipelago 

 but the mountain tops of a sunken continent? To such questions 

 geology gives no definite answer. In some cases, to be sure fjords 

 tell their tale of sunken gorges, or soundings give evidence of a sub- 

 sided coast, with river valleys and former coast-line indicated by sub- 

 marine topography, as in the continuation of the Hudson Eiver valley 

 outward from New York Harbor, and the old shore-line, now at the 

 hundred fathom contour. But these are exceptional cases; and the 

 ocean bed, blanketed with modern deposits, usually gives but scant 

 information to the geologist. 



For the solution of the questions we must address ourselves to 

 another and wholly different inquiry: the geographic distribution of 

 living animals and plants. 



To the pre-Darwinian naturalist, the relationships of animals 

 among themselves and their distribution over the earth's surface were 

 enigmas, quite insoluble upon the hypothesis of special creation. But 

 the doctrine of descent, of the blood relationship of all the members of 

 a genus and family, fills these problems with meaning. If we find that 

 an island, such as England, has the same species of snails, earth- 

 worms, reptiles and fresh water Crustacea and fishes as the neighboring 

 continent, it becomes obvious that there has been a land connection in 

 the past, for there is no other means by which any extensive fauna of 

 these animals could have reached an island. If we take another island, 

 and find that while it has different species from the mainland, yet they 

 belong to the same genera, we must conclude that there has been actual 

 land connection here also, though of more ancient date, across which 

 the ancestors of these transformed species emigrated. 



