842 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
is another instance. Here the surface is at first infolded prob- 
ably for greater strength, but this is carried to a degree so com- 
plex as to be a puzzle to work out. Among the reptiles the 
Stegosaurian family adopted a dermal armor which some mem- 
bers developed till the double dorsal row of plates set on edge 
consisted of pieces some of which are over three feet across and 
several inches thick at the base. With such an excessive load 
of bony weight entailing a drain on vitality, it is little wonder 
that the family was short-ived. Doubtless the elongation of 
the snout was useful to the forms ancestral to Teleosaurus, but 
carried to the excess found in that genus, it resulted in a weak 
grasp, and the form disappeared. 
Among the mammals the saber-toothed tigers have been men- 
tioned. The mammoth with its extreme development of tusks 
may also serve to illustrate the principle. For, while at first the 
moderate tusk is an efficient weapon, as soon as it begins to be 
recurved as in Elephas primigenius, it loses its defensive value, 
and the carrying of the great weight is a drain on the vitality. 
A better example is seen in Babirusa where the canine teeth at 
first developed to protrude outward and of utility in digging, have 
gradually curved up over the snout, and appear more like horns 
than teeth. As they are now and were for a considerable por- 
tion of their previous development, they must be a serious hin- 
drance in feeding. - nm. Lus 
The horns of many forms have also developed beyond useful- 
ness. Take for instánce those of the elk and moose, where if 
useful it is only in conflicts with other "males once a year. But 
their great spread is a constant menace, requiring care lest in 
running they come to trees not far enough apart to admit of 
going between. Then the drain on the animal which sheds 
these great bony growths annually and replaces them again, 
Is great. It is only as the result of momentum that these cases 
seem reasonable. 
R en excessively heavy horns, such as those of 
p and the big-horn, 
weight which lends force in butting c 
to offset the disadvantage which they 
running. Wherever horns have been d 
are developed, the great 
an hardly be considered 
entail in climbing and 
eveloped there seems to 
