THE OLD RED SANDSTONE. 253 
the period passed. With this upper deposit the system 
terminated. - 
Though fish still remained the lords of creation, and fish 
of apparently no superior order to those with which the ver- 
tebrata began at least three formations earlier, they had 
mightily advanced in one striking particular. If their organ- 
ization was in no degree more perfect than at first, their bulk 
at least had become immensely more great. The period had 
gone by in which a mediocrity of dimension characterized 
the existences of the ancient oceans, and fish armed offen- 
sively and defensively with scales and teeth scarcely inferior 
in size to the scales and teeth of the gavial or the alligator, 
sprung into existence. It must have been a large jaw and a 
large head that contained, doubtless among many others, a 
tooth an inch in diameter at the base. I may remark, in the 
passing, that most of the teeth found in the several forma- 
tions of the system are not instruments of mastication, but, 
like those in most of the existing fish, mere hooks for 
penetrating slippery substances, and thus holding them fast. 
The rude angler who first fashioned a crooked bone, or a bit 
of native silver or copper, into a hook, might have found his 
invention anticipated in the jaws of the first fish he drew 
ashore by its means; and we find the hook structure as com- 
plete in the earlier ichthyolites of the Old Red Sandstone as 
in the fish that exist now. The evidence of the geologist is 
of necessity circumstantial evidence, and he need look for 
none other; but it is interesting to observe how directly the 
separate facts bear, in many examples, on one and the same 
point. The hooked and slender teeth tell exactly the same 
story with the undigested scales in the foecal remains alluded 
to in an early chapter. 
In what could this increase in bulk have originated? Is 
there a high but yet comparatively medium temperature in 
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