DISTRIBUTION OF MOLLUSCA IN TIME. 423 



they liarl derived from the occurrence of certain fossil plants, corals, and shells, 

 in high latitudes. 



t The absence of remains of mammalia in the palseozoic formations, is at 

 present a remarkable fact, but it is completely paralleled in the great modern 

 zoological province of the Pacific Islands. 



Barou Humboldt has speculated on the possibility of some laud being yet 

 discovered, where gigantic lichens and arborescent mosses may be the princes 

 of the vegetable kingdom*. If such exist, to shadow the Palaeozoic age, its 

 appropriate inhabitants would be like the cavern-haunting Proteus, and the 

 Sihires which find an asylum eveji in the craters of the Andes. 



V/hat then is it which has chiefly determined the character of the present 

 Zoological provinces ? "What law, more powerful than climate, more influen- 

 tial than soil, and food, and shelter ; nay, often seemingly producing results 

 opposed to a priori probability, and at variance with the suitableness of con- 

 ditions ?t 



The answer is, that each fauna bears, above all things, the impress of the 

 age to which it belongs. Each has undergone a series of vicissitudes up to 

 the time when its barriers become fixed, and after its isolation it has known 

 no further change, but decline. 



As regards the great types of terrestrial organization, their point of com- 

 mon origin seems to have been the centre of the Old World. Here they 

 appear to have been formed in succession, and diffused outwards in all possible 

 directions, to the ends of the earth ; each wave of life developing in its pro- 

 gress special forms adapted to the circumstances of the times, and exempli- 

 fying the modifications of which each type was capable. | 



Chapter IV. 

 ON COLLECTING SHELLS. 



The circumstances under which shells are found is a subject so intimately 

 connected with the methods of collecting them, as to make it undesirable to 

 treat of them separately. 



Naturalists distinguish between the habitats, or geographical localities of 

 species, and the stations or circumstances in which they are found : to the 

 latter subject only slight allusion has been hitherto made. (p. 11). 



Land-shells are most abundant on calcareous soils, (p. 37) and in warm 

 and moist climates. The British species are collected with advantage iti 

 autumn, when full-grown, and showing themselves freelyin the dews of morning 

 and evening. Some species, like halimus acuias, are found only near the sea ; 



* Views of Nature, p. 221. Bohn's ed. t Eurchell, in Darwin's Journal, p. 87. 



X "The Tide of Vegetation has, in the intertropical Pacific Islands, set in a 

 direction contrary to the prevailing winds; namely, from the Asiatic, and not from 

 the American shores." (Hcoker, 1. c. p. 211, note.) 



