954 THE OLD RED SANDSTONE. 
which animals attain their greatest size, and corresponding 
gradations of descent on both sides, whether we increase the 
heat until we reach the point at which life can no longer 
exist, or diminish it until we arrive at the same result from 
intensity of cold? The line of existence bisects on both sides 
the line of extinction. May it not probably form a curve, 
descending equally from an elevated centre to the points of 
bisection on the level of death? But whatever may have 
been the cause, the change furnishes another instance of 
analogy between the progress of individuals and of orders. 
The shark and the sword-fish begin to exist as little creatures 
of a span in length; they expand into monsters whose bodies 
equal in hugeness the trunks of ancient oaks; and thus has 
it been with the order to which they belong. The teeth, 
spines, and palatal bones of the fish of the Upper Ludlow 
Rocks are of almost microscgpic minuteness ; an invariable 
mediocrity of dimension characterizes the ichthyolites of the 
Lower Old Red Sandstone ; a marked increase in size takes 
place among the existences of the middle formation ; in the 
upper the bulky Holoptychius appears; the close of the sys- 
tem ushers in the still bulkier Megalichthys ; and low in the 
Coal Measures we find the ponderous bones, buckler-like 
scales, and enormons teeth of another and immensely more 
gigantic Holoptychius—a creature pronounced by Agassiz 
the largest of all osseous fishes.* We begin with an age of 
dwarfs — we end with an age of giants. ‘The march of Nature 
is an onward and an ascending march ; the stages are slow, 
but the tread is stately; and to Him who has commanded, 
* There have been fish scales found in Burdie House five inches in 
length, by rather more than four in breadth. Of the gigantic Holop- 
tychius of this deposit we have still much to learn. The fragment of 
a jaw, in the possession of the Royal Society of Edinburgh, which 
