264 NATURAL SCIENCE. Oct., 



arctic, while others — Venus chione, Cardinal tuberculatum, and Dentalium 

 taventinum — are generally restricted to more southern regions. 



As far, then, as these particular cases can be taken as typical, 

 we seem to have indications that certain temperate forms of life were 

 able to survive glaciation. 



The present distribution of animals and plants in the British 

 Islands has an interesting bearing on the question. 



That England only possesses a portion, and Ireland a still 

 smaller portion, of the animals and plants of the Continent, is held 

 to indicate the gradual severance of the land connection by which 

 these countries were restocked after glaciation. Ireland is supposed 

 to have been severed from England while the latter was still connected 

 with France and Belgium. It may possibly be, however, that the 

 evidence of these facts of distribution is rather that England was not 

 restocked by a land connection from the Continent after glaciation. 

 If it can be shown that the missing forms are those most likely to have 

 been exterminated by the cold, or least likely to cross the separating 

 sea, and if, in addition, they are forms calculated to migrate as quickly 

 as those which are common to our country, then there will be an 

 argument against the restocking by a land connection. 



The argument of the fifty absent species of mammalia is against 

 the hypothesis of such a land connection, unless it can be shown that 

 there was some other hindrance than lack of time; for the supposition 

 that they had not time will not pass muster: such geographical phases 

 as that implied by the union of *the East Coast of England to the 

 Continent by a great plain are usually of vast duration. 



Supposing the connection to have lasted 10,000 years, + then, to 

 accomplish the journey, they would only need to travel 15, 14,01" 3 yards 

 a year, according as they crossed from the Netherlands to Norfolk, from 

 Belgium to Kent, or by the Straits of Dover. It is difficult to think 

 of any species of mammal which, under favourable climatic and 

 other conditions, would not be able to spread more than from 3 to 

 15 yards per annum. Individuals of most of the species might cross 

 in less than a year. Nor is it easy to believe that any species of 

 reptile would be unable to spread at a much quicker rate than 15 yards 

 a year; and it is to be noted that if the process by which England 

 was united to the Continent was a very gradual one — as such 

 geographical changes usually are — then many species would be half- 

 way across the dividing area by the time the connection was 

 completely established. 



With the plants the case is somewhat different, for they are not 

 able to migrate as individuals like animals. Yet when we consider 

 the numerous means at their disposal — rivers running from the 

 Continent towards the central plain, seeds, like those of the dandelion 



1 According to Mr. Jukes-Browne," it lasted long enough for I'akcolithic man 

 to be supplanted by Neolithic man, and for a large number of mammalia to become 

 extinct." — " The Building of the British Islands," p. 297. 



