24 THE ORIGIN OF VERTEBRATES 
take place until some members of that higher race took again to the 
water, and so competed with the fish-tribe in their own element. 
Another most important conclusion to be derived from the 
uprising of the Amphibia is that at that time there was no race 
of animals living on the land which had a chance against them. No 
race of land-living animals had been evolved whose organization 
enabled them to compete with and overcome these intruders from 
the sea in the struggle for existence. For this reason that the 
whole land was their own, and no serious competition could arise 
from their congeners, the fish, they took possession of it, and increased 
mightily in size; losing more and more the habit of going into the 
water, becoming more and more truly terrestrial animals. Hence- 
forth, then, in trying to find out the sequence of evolution, we must 
leave the land and examine the nature of the animals living in the 
sea ; the air-breathing animals which lived on the land in the Upper 
Silurian and Devonian times cannot have reached a stage of organi- 
zation comparable with that of the fishes, seeing how easily the 
amphibians became dominant. 
We arrive, then, at the conclusion that the ancestors of the fishes 
must have lived in the sea, and applying still the same principles 
that have held good up to this time, the ancestors of the fishes must 
have arisen from some member of the race predominant at the time 
when they first appeared, and also the earliest fishes must have much 
more closely resembled the ancestral form than those found in later 
times or at the present day. 
What, then, is the record of the rocks at the time of the first 
appearance of fish-like forms? What kind of fishes were they, and 
what was the predominant race at the time? 
We have now reached the Upper Silurian and Lower Devonian 
times, and most instructive and suggestive is the revelation of the 
rocks. Here, when the first vertebrates appeared, the sea was peopled 
with corals, brachiopods, early forms of cephalopods, and other in- 
vertebrates ; but, above all, with the great tribe of trilobites (Fig. 6) 
and their successors. From the trilobites arose, as evidenced by 
their larval form, the king-crab group, called the Xiphosura (Fig. 5). 
Closely connected with them, and forming intermediate stages 
between trilobites and king-crabs, numerous forms have been dis- 
covered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc. 
(Fig. 5 and Fig. 12). From them also arose the most striking group 
