]6l 



PSYCHE. 



1 February, rgoi 



about and feed upon dead nioUusks ; as 

 the tide rises they, hide under stones and 

 become submerged. 



Now this species occurs not only on 

 the coasts of New England, Long Island, 

 Florida, and probably of intervening 

 places, but also along the entire western 

 coast of Europe ; its distribution by 

 marine currents, therefore, cannot be 

 doubted. 



Isotoma bessehi, a marine species 

 found first at Polaris Bay, is not rare on 

 the coast of Massachusetts and has late- 

 ly been found in Spitzbergen. Xcnylla 

 humicola has a similar distribution. 



Achoruta lullbcrgi {(iiih'nis TuU.) in- 

 habits Siberia, Nova Zembla, Franz 

 Josef Land and Spitzbergen, and also 

 occurs on the salt marshes of eastern 

 Massachusetts. 



Achorutes viatictis not only ranges 

 over the Palaearctic region, including 

 the arctic islands, but has been found 

 in California and even in Tierra del 

 Fuego. 



Finally, Achorutes armatiis, which 

 occurs throughout the Holarctic region, 

 has been taken in South America, New 

 Zealand and Sumatra. 



More cosmopolitan forms are known, 

 but none of the other faunal regions 

 share their species to the extent to 

 which the Palaearctic and Nearctic do. 



I have shown that a number of gene- 

 ralized species of Collembola inhabiting 



both Europe and the United States are 

 practically continuous in their distribu- 

 tion between the two places. The fact, 

 however, that most Holarctic species 

 are discontinuous must be accounted 

 for and the explanation of the fact is 

 important for its general bearing. 



Nearly all the Holarctic species of 

 the Arctic regions proper belong to the 

 most generalized families, i. e., Aphor- 

 uridae and Poduridae, and can live in 

 an environment of extreme simplicity 

 and rigor, needing but the scantiest of 

 vegetation and being quite tolerant of 

 cold ; in temperate regions they are 

 noticeably the most active species of 

 the order during the winter. 



The Entomobryidae and Sminthuridae, 

 on the other hand, require more warmth 

 and certainly a more luxuriant vegeta- 

 tion than arctic regions afford. Now 

 these most specialized families, adapted 

 to a more complex environment, contain 

 many species which, although identical 

 in middle Europe and the United 

 States, do not exist, it may safely be 

 said, in the intervening regions, where, 

 most probably, they formerly must have 

 occurred. The inference is obvious, 

 then, that a higher temperature and 

 a more luxuriant vegetation than at 

 present once prevailed in the arctic 

 zone. This is no news, of course, — 

 but the additional evidence is worth 

 something. 



