2HH THK ICK A(!E IN CANADA. 



materially aflected, ami they liavn had to make iniporlant 

 changes of habitat, while .some of them iiave so extended 

 their range as to be found on both sides of the North 

 Pacific anil North Atlantic. Vet all the Pleistocene 

 species survive, and this without change. Even variable 

 forms like the species of Jiaccinum and Astarie show the 

 same range of variation in the Pleistocene as in the 

 modern, and though some varieties have changed dieir 

 geographical ])osition, they have not changed their 

 character. These changes of geographical jwsition are 

 also very significant, as they seem to show that arctic and 

 temperate varieties are readily convertible into each other 

 when the temperature (tf the water changes, but revert to 

 the old forms on restoration of the old conditions.* This 

 result is obviously independent of imperfection of the 

 geological record, because there is no reason to doubt that 

 these species have continuously occupied the North 

 Atlantic area, and we have great abundance of them for 

 comparison both in the Pleistocene and the modern seas. 

 It is also independent of any questions as to tiie limits of 

 species and varieties, inasmuch as it depends on careful 

 comparisons of the living and fossil specimens ; and by 

 whatever names we may call these, their similarity t»r 

 dissimilarity remains unafifected. We have at present no 

 means of tracing this fauna, as a whole, farther back. 

 Some of its members we know existed in the Pliocene and 

 Miocene without specific difference ; but some day the 

 middle tertiaries of Greenland may reveal to us the 

 ancestors of these shells, if they lived so far back, and 

 may throw further light on their origin. In the mean- 

 time we can affirm that tiie lapse of time since the Pliocene 



* See above, the remarks on the species of Mya. 



