SQUIRKELS. 261 



Two questions of interest arise out of the close resemblance of this and other species similarly 

 circumstanced, such as Tamias striatus and T. Pallasii. The one is, whether the_y are identical or 

 not ; and the other, to what cause their extreme resemblance or identitj' is due. As to the first 

 question, that is a matter of opinion, which every one will answer according to his own views of 

 what constitutes a species ; they hover on the borders between a species and a variety. As to 

 the second question, the explanation of their sujiposed identity is generally assumed to be, that in- 

 dividuals have crossed over from the one continent to the other by Bhering's Straits, when the 

 sea there was frozen over in winter. 



Dr. Baird says, and I suspect the majority of naturalists would concur in the remark, that 

 "there is nothing to prevent the mammalia of the north-western portions of the American 

 continent from passing over to Asia, as the strait intervening is frozen solid every winter."* 



Now I think wo ma}' sometimes stretch this idea too far. It is perfectl}' true as regards 

 some animals, but I am not so sure that it is equally ajiplicable to all. The quotations wliich I have 

 already given from Captain M'Clintock's Diary show that apparently the whole of Polar animal 

 life is migratory, and swarms across the ice as freelj^ as over the land. But I have some difiiculty 

 in believing that non-Polar animals would equally avail themselves of the same means of transit. 

 Certainly the fact seems to have been that these Spermophiles do not avail themselves of this yearly 

 bridge, for neither of them extend their range beyond their respective continents, but come up 

 close to the Straits, S. Parryi being found in the island of Aricamtchitchi at the Straits, and 

 the other on the Kamtschatkan shore. If it were not so — if the time when they crossed from 

 one continent to the other was not distant, why are they not absolutely identical ? If S. Parryi 

 could take a run across to see its cousin S. Eversmaxni every winter, or S. Eversmanni in like 

 manner come over to America, why should all the American specimens be bigger ? and why 

 should they all, and always, have shorter and bushier tails ? Some distinctive difference occurs in 

 every species with which I am acquainted, which is represented both in North America and the 

 Old World. There is constantly a perceptible distinction, altliough it be slight. On this 

 ground it seems to me plain that the journey across Bhering's Straits is not a thing which 

 " there is nothing to prevent." In one sense (the physical one) there is nothing to prevent it. 

 So there was nothing to prevent any one before Columbus sailing from Europe to America ; 

 and if a Squirrel had the intelligence, ambition, and perseverance of Columbus, it no doubt 

 woiUd soon cross the Straits. But not having them, what is there to induce it to leave the land 

 where its food is, and to start on a journey of sixty miles across a frozen sea Pf nay, not across, 

 but, to all its perception, on an illimitable horizon of ice, without bourne, and without object or 

 inducement, but opposed to everything of the kind. /Vndlet the reader think for a moment what sixty 

 miles is. He is, perhaps, a good walker, and when in fair training will walk his thirty miles a 

 day. It would take him two days to cross the Straits, sleeping one night on the ice ; but if he only 

 made out ten miles, which woidd probably be enough for the energy of a Spermojihile, then he 

 must ■'sleep another night and walk another day ; and again, and again, and again, and yet again, 

 before he reached the oj)posite shore. It is not as if they were in a boat, which, once set adrift, 

 might be blown hundreds of miles without effort on the part of those in it. They must set out 



* Baird, op. cit. p. 324. to find a reliable statement of their width, but the tnea- 



t Sir Charles Lyell says that Bhering's Straits do not suroment by scale ou our maps gives sixty miles. 

 exceed in width the Straits of Dover. I have been unable 



