AID IN COLLECTING. 519 



some of his most intelligent and sympathetic 

 listeners in the working class. Now that he 

 needed their assistance he often found his co- 

 laborers among farmers, stock-raisers, sea-far- 

 ing men, fishermen, and sailors. Many a New 

 England captain, when he started on a cruise, 

 had on board collecting cans, furnished by 

 Agassiz, to be filled in distant ports or nearer 

 home, as the case might be, and returned to 

 the Museum at Cambridge. One or two let- 

 ters, written to scientific friends at the time the 

 above-mentioned circular was issued, will give 

 an idea of the way in which Agassiz laid out 

 such investigations. 



TO JAMES D. DANA. 



Cambridge, July 8, 1853. 



... I have been lately devising some 

 method of learning how far animals are truly 

 autochthones, and how far they have extended 

 their primitive boundaries. I will attempt to 

 test that question with Long Island, the larg- 

 est of all the islands along our coast. For 

 this purpose I will for the present limit myself 

 to the fresh-water fishes and shells, and for 

 the sake of comparison I will try to collect 

 carefully all the species living in the rivers of 

 Connecticut, New York, and New Jersey, and 



