42 
Gravitation as a Factor 
[January, 
Turning to the sub-kingdom of the Vertebrates, popularly 
so much better known, we find in its highest class, the 
Mammalia, the largest of all known animals, the whales, 
all stridtly aquatic. Several of the seals— such as the 
bottle-nose, which reaches a length of 25 feet — and the 
walrus also far exceed the average stature of land-animals. 
Nay, amongst those ordinarily classed as true terrestrial 
forms, many of the largest — such as the hippopotamus, the 
elephants, the various species of rhinoceros, even the true 
buffalo ( Bubalus bubalis) are marsh-dwellers, semi-aquatic 
in their haunts and habits. The largest of the cats, the 
Bengal tiger, is also the most natatorial. 
Among the reptiles we find a state of things closely 
analogous. Of the Saurians at present living no terrestrial 
species can at all compare with the crocodiles. Among the 
gigantic extindt groups the majority at least — such as the 
Ichthyosauri, the Plesiosauri, and the Pythonomorphse (if 
the latter are not rather Ophidian) — must have been 
aquatic. 
If we examine the serpents we find, again, that the 
largest species are aquatic, — as, for instance, Eunectss mu- 
rinus, — inhabiting the rivers of South America. 
Among the Chelonians the same rule prevails, the turtles 
being certainly much larger than their terrestrial kindred, 
the tortoises, even those of the Gallapagos and the Mas- 
carenes. 
Of the Mollusca only a minority are terrestrial. But 
here also we find the smaller species inhabiting the land, 
and the larger forms, especially the gigantic cuttlefishes, 
confined to the waters.* 
I may here remark that Swainson, though setting out 
from totally different considerations, and though accepting 
each animal species as an independent ultimate fadt, lays a 
decided emphasis on the magnitude of “ natatorial or 
aquatic types,” which he pronounces “ as to structure, 
chiefly remarkable for their enormous bulk.”t 
In opposition to the vast growth of the aquatic groups, 
we find the inhabitants of the air for the most part diminu- 
tive. The bulkiest birds are puny in comparison with the 
bulkier members of the other vertebrate sections. Further, 
even among birds, the largest species — such as the ostrich, 
emeu, and cassowary — are stridtly terrestrial in their habits, 
as was also the gigantic extindt moa ( Dinornis ) of New 
Zealand, and in all probability the still larger JEpiornis of 
* Cephalapods are known to reach the weight of 2 tons, 
f Geography and Classification of Animals, p. 249, § 309. 
