Chap. XX ] GEOGRAPHICAL RANGE OF PRIMEVAL LIFE. 
503 
hemispheres. Assisted by Mr. Salter and Mr. Etheridge, he has registered 
no less than 7553 well-defined species of fossils that lived during the 
Silurian epoch, thereby showing that in the last ten years the number of 
known species has been more than trebled. "When, indeed, we consider 
that through the enlightened researches of Barrande, a small tract around 
Prague has already yielded a third of the hitherto recognized Silurian 
remains, we may well imagine what will be the number of the fossil 
population of that age to be cited by the geological statists of the next 
century. A glance at the Tables of the ' Thesaurus Siluricus ' will 
show the groups of fossils which are common to the most distant centres ; 
and we thus ascertain that some of the species even are common to Britain, 
Scandinavia, Bohemia, America, and the Antipodes in Australia. These 
data prove that the Silurian system of life had an almost universal spread; 
for the knowledge we have obtained has been gathered, as the work 
well reminds us, from widely separated spots only on the surface of the 
earth, the greater mass of such old rocks being hidden beneath the 
younger formations. Another remarkable result of the researches of 
this geologist is the amount of the frequent recurrence of species in the 
range of Silurian life. Thus, in pursuing the method of analysis and 
comparison first applied by Professor John Phillips to the Carboniferous 
era, Bigsby enumerates no less than 803 species (or 12 per cent, of the 
whole number) which have maintained their existence during vastly 
long periods, though after all 6200 species out of 7553 are severally 
restricted to one horizon. Equally striking is the generalization given in 
this * Thesaurus ' regarding the geographical distribution of Silurian life, 
particularly as exhibited in the grand natural features of British North 
America, where the formations of that period extend over an enormous 
area, as well shown in the map of Canada and neighbouring States by 
Logan and his associates, referred to in Chapter XVIII. 
Eesting then on these universal facts as a firm basis, the geologist 
who explores his way upwards sees, as before stated, that the formations 
which were next accumulated in the same latitudes as the Silurian rocks, 
and sometimes in actual and conformable contact with them, do contain 
Land Plants mixed with marine remains. In short, the only prevailing 
unequivocal vegetables found in the oldest strata are Sea-weeds ; whilst the 
after-formed and contiguous rocks, though equally charged with exuviae of 
the sea, are laden with many spoils of the land, both vegetable and animal. 
Patient researches having thus demonstrated that in the primeval 
eras all living things differed from those of our own times, we also see 
how the animals subsequently created were adapted to new and altered 
physical conditions. Proceeding onwards from the early period in which 
we can trace no sign of Trees on the land, or of Fishes in the sea, and in 
which the solid materials, enclosing everywhere a similar fauna, were 
widely spread out with great uniformity, we soon begin to perceive that, 
