LINKS BETWEEN EXISTING SPECIES. 307 



of which thirty-four at present live in Sweden,* and twenty- 

 nine in Lombardy. These latter, however, are principally 

 species having a very wide range, and we shall see still more 

 clearly that the leaning of the molluscan fauna is towards the 

 north, if we remember that out of seventy -seven Finland 

 species, thirty-one have been found in the upper level gravels, 

 while of 193 Lombard species, only twenty-nine have as yet 

 occurred. 



A very interesting point connected with this quaternary 

 fauna is the manner in which it connects together species 

 now quite distinct. Opponents of Mr. Darwin's theory often 

 ask with misplaced triumph for the links connecting any two 

 species. In fact, however, every species is a link between 

 other allied forms. Of course, indeed, as long as any varie- 

 ties remain undescribed there will be intervals indicating 

 however, gaps not in nature, but in our knowledge. More- 

 over^ it is admitted by every one that there are variable 

 species, that is to say, species which present two or more 

 extreme forms, with intermediate gradations. Now we may 

 fairly ask those who assert that no two species are connected 

 by links, how they would separate the instances of variable 

 animals (which they admit to occur) from the case which 

 they say does not exist. If we were to obtain to-morrow all 

 the links between any two species which are now considered 

 distinct, no one can deny that the two would at once be 

 united, and would hereafter appear in our classifications as 

 one variable species. In fact, therefore, they first unite into 

 one species all these forms, however different, between which 

 a complete series is known, and then argue in favour of the 

 permanence of species because no two of them are united by 

 intermediate links. 



Moreover, if species were in reality perfectly distinct from 

 one another, then it would necessarily follow that, as our 



* Proc. Roy. Soc, 1862, p. 44. 

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