water which surrounds their food must blunt the senses of taste and smell, and there is little besides water contact to develop touch.
Also the conditions surrounding attack and escape are here greatly simplified. The water presents no lurking-places, no ambush, except to the inactive dwellers upon the sea-bottom. And, by relieving its inhabitants from the effects of gravitation, it renders rapid motion easy, with slight muscular exertion. Thus the easiest, most natural, and most effective means of assault and defense is by swift powers of swimming.
The same principle holds good in the case of the dwellers of the air. There is here no lying in wait for prey nor hiding from assault. Flight is the most effective and most ready means of attack and escape, and the only one available in the fields of open air. Thus purely water and purely air animals fail to develop a variety of resources in this particular.
Moreover, as fish are at a disadvantage from their imperfect oxidation, so are birds at a disadvantage from the effects of gravitation. The weight of fishes is almost or entirely supported; that of land animals partly supported; that of birds almost unsupported. Gravitation in them, then, must be mainly overcome by muscular exertion. Thus a large proportion of their life-force is exhausted by the effort to sustain themselves against the constant downward pull of gravity. This places them at a disadvantage with animals capable of using their forces for more varied purposes.
Indeed, we find, both in fishes and in birds, a tendency to avail themselves to some extent of the advantages which the land-surface gives. And those that most display this tendency comprise the species of most varied resources, and with the greatest degree of functional adaptation. Many species of birds, in fact, have found safety so much more assured on the land, that they have lost, first their instinct, and then, in some cases, their power of flight, and have become, virtually, surface-animals.
We may descend to the invertebrate world for one marked instance of this. The ants are acknowledged to be the highest of all insects in functional development. Their eyes, for instance, are more simplified than those of other insects; and, if we class mental attributes as nerve-functions, their claim is indisputable. Yet they have forsaken the air and taken to the earth in preference, the females casting off their wings as if in scorn, after a temporary use of them.
Of all the fields of life for the varied display and functional separation of the animal forces, we are thus brought to the land-surface as obviously the best. Here an abundant supply of oxygen assures vital activity; the firm ground largely supports the weight, and releases the muscular powers for employment in other directions; the abundance and variety of vegetable food sustains vast numbers of animals; the great diversity of conditions causes wide specific variation; while
