GEOGRAPHICAL DISTRIBUTION — PAST AKD PRESENT. 219 



case, however, for, with an equable temperature over the greater 

 part of the earth's surface, it does not appear why species, or 

 groups of species, should not have readily found their way to 

 the' most distant localities by simply following the line of coast. 

 The marine Antarctic fauna of the present time is more nearly re- 

 lated to the Arctic than to any other, and, if the species of the two 

 are almost altogether different, yet a large proportion of the generic 

 types, wanting in the intermediate region, are the same. Such are, 

 for example, the genera Trophon, Buccinum, Margarita, Astarte, 

 Admete, &c. A limited number of the species, too, are identical 

 with species occurring in the waters of the north Temperate Zone. 

 There can be little doubt that the distribution of these forms to the 

 antipodal regions of the earth's surface was effected by way of the 

 deeper zones of cold water, and there seems to be no necessity for 

 invoking the aid of a " spontaneous " Antarctic fauna to account 

 for the separation that exists between it and the Arctic. If this 

 explanation is the correct one, it is then a little surprising that we 

 do not meet, in the two regions, with a larger number of identical 

 forms; but it can readily be conceived that the necessary accom- 

 modation to new conditions of existence, imposed by pressure, 

 absence of light, and a different food-supply, may have brought 

 about, in the course of such a lengthy migration, variation in specific 

 characters, without sensibly interfering with the structural type of 

 the group. 



If it now be assumed that, during the Paleozoic era, the broad 

 distribution of species was effected in pretty much the same manner 

 — i. e. , by following the trough of the sea rather than the continental 

 border — we are at once confronted by the anomaly that the num- 

 ber of identical species occurring at the most widely separated 

 localities, instead of being very limited, is just the reverse. It is 

 true that, with a high temperature extending to the bottom of the 

 sea, specific modification resulting from a transference of abode 

 from the surface to the greater depths may have been less marked 

 than appears to be the case with a low temperature ; but we have 

 no reason for supposing that any great variation in this respect 

 did exist, and almost certainly not enough to account for the dif- 

 ferences that present themselves. It would thus seem that the 

 broad distribution of former periods was effected principally along 

 predetermined coast-lines. One circumstance, however, which has 



