100 TESTS OF THE DIFFERENT AGES [Ch. IX 



rated from each other by very narrow limits ; and hence it must happen, 

 that strata will be sometimes formed in contiguous regions, differing 

 widely both in mineral contents and organic remains. Thus, for exam~ 

 pie, the testacea, zoophytes, and fish of the Red Sea are, as a group, ex- 

 tremely distinct from those inhabiting the adjoining parts of the Mediter- 

 ranean, although the two seas are separated only by the narrow isthmus 

 of Suez. Of the bivalve shells, according to Philippi, not more than a 

 fifth are common to the Red Sea and the sea around Sicily, while the 

 proportion of univalves (or Gasteropoda) is still smaller, not exceeding 

 eighteen in a hundred. Calcareous formations have accumulated on a 

 great scale in the Red Sea in modern times, and fossil shells of existing 

 species are well preserved therein ; and we know that at the mouth of 

 the Nile large deposits of mud are amassed, including the remains of 

 Mediterranean species. It follows, therefore, that if at some future pe- 

 riod the bed of the Red Sea should be laid dry, the geologist might ex- 

 perience great difficulties in endeavoring to ascertain the relative age of 

 these formations, which, although dissimilar both in organic and mineral 

 characters, were of synchronous origin. 



But, on the other hand, we must not forget that the northwestern 

 shores of the Arabian Gulf, the plains of Egypt, and the isthmus of 

 Suez, are all parts of one province of terrestrial species. Small streams, 

 therefore, occasional land-floods, and those winds which drift clouds of 

 sand along the deserts, might carry down into the Red Sea the same 

 shells of fluviatile and land testacea which the Nile is sweeping into its 

 delta, together with some remains of terrestrial plants and the bones of 

 quadrupeds, whereby the groups of strata, before alluded to, might, not- 

 withstanding the discrepancy of their mineral composition and marine 

 organic fossils, be shown to have belonged to the same epoch. 



Yet while rivers may thus carry down the same fluviatile and ter- 

 restrial spoils into two or more seas inhabited by different marine species, 

 it will much more frequently happen, that the coexistence of terrestrial 

 species of distinct zoological and botanical provinces will be proved by 

 the identity of the marine beings which inhabited the intervening space. 

 Thus, for example, the land quadrupeds and shells of the south of Eu- 

 rope, north of Africa, and northwest of Asia, differ considerably, yet their 

 remains are all washed down by rivers flowing from these three countries 

 into the Mediterranean. 



In some parts of the globe, at the present period, the line of demarca- 

 tion between distinct provinces of animals and plants is not very strongly 

 marked, especially where the change is determined by temperature, as it 

 is in seas extending from the temperate to the tropical zone, or from the 

 temperate to the arctic regions. Here a gradual passage takes place 

 from one set of species to another. In like manner the geologist, in 

 studying particular formations of remote periods, has sometimes been 

 able to trace the gradation from one ancient province to another, by ob- 

 serving carefully the fossils of all the intermediate places. His success 

 in thus acquiring a knowledge of the zoological or botanical geography 



