45 



explain the causes resulting in the present peculiar distribution, I 

 think it does throw considerable light upon the relationship between 

 environment and present distribution. Since arriving at this 

 conclusion, I have chanced to note that in several British works, 

 considerable attention is paid to the fact that the Hart's-tongue 

 occurs regularly " in caves, on the seashore and in other cold and 

 damp situations, " and again, "more especially about the mouths 

 of caves, deserted mines, at the borders of wells, where there is a 

 current of cold or moist air." 



It is, of course, a well known fact that plants characteristic of 

 high northern latitudes, are found in more or less abundance upon 

 mountain peaks of the more temperate regions thousands of miles 

 to the south. The accepted explanation is : that during the glacial 

 epoch the plants, gradually forced south by the advancing ice sheet, 

 upon the northward retreat of the ice, moved up the mountains, 

 seeking to maintain accustomed environmental conditions, * or, for 

 the same reason, advanced to the northward. Thus, the Hart's- 

 tongue occurs in America mostly in the north. It has, as I have 

 remarked, been usually regarded as a boreal type. I believe that 

 it was once far more common than at present, and that it will fre- 

 quently be found in the north, perhaps, as I have suggested, 

 stretching across from Asia to Alaska, and down the Pacific coast. 

 In the United States it has yet to be seen west of the Mississippi, 

 though it may possibly occur along the upper tier of States, assum- 

 ing that it follow a belt parallel to the lower limit of glaciation. 

 Especial search and exploration in favorable localities would not 

 be without good results in general, and would very likely result in 

 further extensions of range for what has commonly been regarded 

 as one of the rarer American species of ferns. 



The distribution of the Hart's-tongue in Great Britain is pecu- 

 liar as well, and has been commented upon by Mr. Druery (Choice 

 British Ferns, p. 14. 1888) at some length. He remarks that its 

 comparative rarity in Scotland is the more unexpected in view of 

 the " innumerable glens which abound there and seem a very beau 

 ideal of a habitat for it;" and adds, moreover, "this fern is one of the 



* All especially interesting exposition of the facts and causes having to do 

 with the distribution of species (especially North American), is contained in 

 the latter portion of Dr. Asa Gray's " Memoir on the Botany of Japan, etc," in 

 Mem. Am. Acad. Arts and Sci. II. &' l8 59- 



