MACKIE — FIRST TRACES OF THE SUCCESSION OF 1,IFE. 
mostraca) abound ; fish next appear in number ; then reptiles reign ; 
then dawns the era of the gigantic mammals ; and then the Age of 
Man sets in. So in vegetation, as far as we can judge from the 
strange and singular fossil-forms presented to our view, the flower- 
less preceded the flowering plants and trees which so luxuriantly 
covered the Tertiarj'' lauds, and still adorn our ovai. 
But this remarkable advance, so evident when we regard the 
grander groups and the results of ages as a whole, becomes less 
apparent and indeed very obscure when we attempt to combine the 
seeming links of minor details and to trace one form developing 
itself into another. We see the age of reptiles succeed that of fish ; 
the age of man following on that of the lower mammals ; and we 
have no difficulty in appreciating the higher stages of each, and the 
successively improved conditions of our planet to which they were 
adapted; but Avhen we attempt to trace out links to join the reptile 
with the fish, and the quadruped mammal with the man, we fail. 
We may see resemblances of development on either hand, but no 
true junction-forms ; and then again when, as between genera and 
families, we do meet with connecting species, such may occur in time 
either in advance or in arrear of the age or ages in which the forms 
so connected existed ; while, on the other hand, some genera, such 
for example as the Lingula we meet with for the first time in the 
Lower Silurian rocks, have lasted from their first appearance to the 
present hour -with scarcely more than a specific difference between 
those primitive individuals and those now living in our seas. One 
thing, however, seems certain of the Lower Silurian mollusca, Crus- 
tacea, and annehdes, that their geogTaphical distribution was far 
greater and much more universal than is the case with existing 
species ; and although lines of demarcation have been attempted to 
be draA\'n between many of the American Lower Silurian fossils and 
those of onr own countiy, it is very questionable, at least in some 
cases, if any real specific distinctions exist. 
The general appearance and character of the Stiper-stones have 
been well described by Sir Roderick Murchison, in whose wake, so 
great has been his own energy and so powerful the means at his dis- 
posal, a British author wi-iting on Silurian strata, is almost com- 
pelled to follow. " Trending in a broken mural line from north- 
