xliv FACTS AND OBSERVATIONS. 



The Isozonals in the Table U are more than double the number of the migrants, these 

 latter being somewhat few, because of the distance traversed, and because this is only one of many 

 such lines of molluscan journeying. 



We remark that the number of migrants from America to Europe is less than that of those 

 going in the opposite direction, so that the exchange is against the West an unexpected and dis- 

 appointing conclusion ; and we see that the orders likely or unlikely to travel behave as might have 

 been expected, Coelenterata, Trilobita, and Brachiopoda being the most active of all, while the two 

 last, together with Polyzoa, most abound with isozonal species common to the two hemispheres. 

 This Table contains six orders having no species making this traverse, not even the Dimyaria, an 

 order holding 526 species. 



In 1866-7, before I received further contributions from M. Barrande, the species registered 

 were 7767, and the appearances were 10,447. But as it has already been explained that we have to 

 do only with half the former number, it results that every species which moved at all passed into 

 two other countries, or nearly (3883 species, 10,447 appearances), because the appearances were in 

 very numerous instances not set down for want of space. 



But this statement affords a very imperfect view of Silurian migration or dispersion ; for a 

 certain number of species of almost every order are planted east, west, south, and north, in 5, 7, 10, 

 15, 22 countries, almost belting the world; whether by radiation from a common centre, or by 

 ordinary migration, is not yet known. 



This is true of the quasi-universal species already mentioned, and of many others. As a rule, 

 but with exceptions, this scattered fauna is always on the same stratigraphical level, whether that be 

 of the lower or of the upper stage. 



A Montreal fossil we trace southwards to Pennsylvania, westwards into Minnesota, and east- 

 wards to Anticosti ; Minnesotans are found in the Texas &c. The Australian Diplograpsus pristis 

 (Hisinger) also nourished in Britain and Canada on the same zone. 



The Illeenus crassicauda, Favosites Gothlandica, several Leptcena, Orthides, and other genera 

 mark with their presence lines 6000 miles long, attracting notice first in Canada, Russia, or 

 Britain, &c. 



RECURRENCE OR VERTICAL RANGE. What can be more unexpected or more wonderful than the 

 upward passage of a mollusk by successive generations, through stages and epochs, during centuries 

 almost countless ? What a vast train of descendants must have followed the first ancestor ! And 

 it is a fact which grows in importance as we ascend the sedimentary column to its present summit. 

 The doctrine of limited duration in species must sometimes require an elastic interpretation. 



" Recurrence," a phrase of one of my masters, Prof. John Phillips, is simply the reappearance 

 of a plant or animal in a zone of rocks higher than that in which it was first observed. It implies 

 progress upwards, either on the same spot or on another by migration. Instances of both kinds 

 are plentiful. 



Recurrency is the more worthy of our attention, because Edward Forbes * thinks " that there 

 often prevails an extreme and unwholesome tendency on the part of many palaeontologists to insist 

 on the real distinctness of the species found in different stages, and to force their diagnoses accord- 

 ingly." A few years ago many of our best naturalists forbade, the belief in vertical range, except in 

 rare cases M. Agassiz asserting that the number of supposed instances was daily diminishing with 

 advancing knowledge ; but a far greater latitude is now very generally granted to this operation. 

 Thomas Davidson, F.R.S., a very high authority, remarks f, " It is now acknowledged that many 

 species have lived through several stages of the Silurian system, and are even perpetuated beyond 

 it; and this applies equally to palaeozoic and Jurassic fossils. To narrow too strictly the strati- 

 graphical limits of species is to expose ourselves to adopt even false and puerile characters in fossils 



* Quart. Journ. Geol. Soc. Lond. vol. x. p. 40. f Bull. Soc. G6ol. de France, vol. v. n. s. p. 310. 



