$8 PENIKESE. 



themselves by describing new species. Now if evolu- 

 tion be true down will go their species." (The profess- 

 or had been talking particularly of mollusks, but 

 whether of the Unios or of the Land shells, the notes do 

 not state). "Look at the difference in the number of 

 species abroad and in this country. In England they 

 have been reduced to about forty; and an eminent nat- 

 uralist has taken some dozen of these and, by subject- 

 ing them to different circumstances, actually reduced 

 that number. If this can be done in a short period 

 of time, what might we not expect in looking back for 

 one or even two hundred thousands of years? Take 

 the species in New England, some twenty; and then 

 go to the western portion of the United States, in 

 Ohio, Tennessee, and the tributaries of the vast Miss- 

 issippi surrounded on the north by the Laurentian 

 chain, on the east by the Alleghanies, and on the west 

 by the Rocky mountains one vast basin! aud we find 

 species living in the brackish pools of water that are 

 identified with those living in the sea. In the fresh 

 water we find living species that are identical with 

 those living in the brackish water. These all go by 

 different names, because found in different localities; 

 but they are so absolutely identical that, if placed 

 side by side it is impossible to separate them. This 

 is true of a great majority of our species so called. 

 Now what is the cause? It is clear enough to my 

 mind. The sea, formerly, filled this large basin, and, 

 gradually receding, left those large rivers which, af- 

 ter being nourished by the rain for years, grew fresh 

 retaining many of their old and more hardy forms of 

 animal life from the ocean. These forms, in turn, 

 gradually became changed so as to sustain life in 

 brackish, and then in fresh water. This process of 

 change of living to suit a difference of environment, 

 can now actually be performed artificially with some 

 of our species. Thus we find, that it is possible to 

 take forms which if introduced directly into fresh wat- 

 er would be instantly killed, and by gradually modify- 

 ing their circumstances, cause them to become actual 



