Chap. XX.] 
SILURIAN LIFE. 
477 
regions. Proceeding upwards from the earliest of these zones, we then 
ascended to other sediments, in which we recognized a more copions dis- 
tribution of marine creatures, closely resembling each other, though im- 
bedded in rocks separated by wide seas, and now often raised up into 
the loftiest mountains. Examining all the strata exposed to view that 
were formed during the first long natural epoch of the life which I 
termed ' Silurian,' we found that the successive deposits were charged with 
a great variety of forms — such as the Trilobite or primeval Crustacean, 
with a few of the earliest Chambered Shells, as well as numerous exqui- 
sitely formed Mollusks, Crinoids, and Zoophytes, — the families of Cystideans 
and Graptolites being exclusively found in these Silurian rocks. In short, 
examples of every group of purely aquatic animals, save Fishes, have been 
assembled from those ancient Lower Silurian sediments. 
Though we are now as well acquainted with the contents of the Palaeo- 
zoic rocks as with those of Mesozoic age, the multiplied researches during 
the last thirty-two years have failed to detect the trace of a Pish, amid 
the multitudes of marine beings, in the various sediments which constitute 
the great mass of Silurian rocks. Of these animals, though they are the 
lowest in the scale of the great division ' Yertebrata,' we are unable to 
perceive a vestige until we reach the ' Ludlow' zone of the Upper Silurian, 
and are about to enter upon the Devonian period. In fact, when I last 
wrote, the few Cartilaginous Pishes of the uppermost Silurian forma- 
tion still remained the most ancient known beings of their class* (see 
PI. XXXV.), — a generalization which was first established by my own 
researches f in 1835. 
Looking, however, at the Silurian System as a whole, and judging from 
the collection of facts gathered from all quarters of the globe, we know 
that its chief deposits (certainly all the lower and by far the most exten- 
sive) were formed during a very long period in which, while the^sea 
abounded with countless invertebrate animals, no Bony Pishes had been 
called into existence. The Silurian (except at its close) was consequently 
an epoch in which there appeared no example of that complete bony 
framework in which, as approaching to the vertebrate archetype, the 
comparative anatomist traces the first step in that series of creations 
which ended in Man J. 
"Whether, therefore, the term ' successive ' or ' progressive ' be applied 
to such phenomena, I assert that all the evidences derived from close and 
long-continued researches have demonstrated that there was an enormous 
* The only addition to our knowledge on this gression in creation, in the Quarterly Keview, 
point since I made the announcement of 1835 is, 1851, p. 412 et seq. The arguments there em- 
that the genus Pteraspis has been found in the ployed have been strengthened by subsequent 
Lower as well as in the Upper Ludlow rock. discoveries alluded to in this volume. I would 
t See ' Silurian System,' p. 605. also specially refer the reader to Professor Sedg- 
I See Owen on the Homology of the Verte- wick's ' Discourse on the Studies of the University 
brate Skeleton, Keports Brit. Assoc. Adv. Science, of Cambridge ' for a masterly and eloquent illus- 
1846, p. 169. The general reader will find a power- tration of several of the views which are here 
ful essay, embodying the opinions, I believe, of advocated, 
the same high authority, on the proofs of a pro- 
