The connection between zoological rank and the absence of metamorphosis is also explained by what we see among men. High zoological position ordinarily implies strength or intelligence, and the strong and knowing can do better for their offspring than the puny and sluggish. It does not cost a Shark or a quadruped too much to hatch its young in its own body, while Spiders and Earwigs,[187] which are among the highest Invertebrates, defend their progeny, as do Mammals and Birds, the highest Vertebrates.
But what has all this to do with habitat? Are fluviatile and terrestrial animals, as a rule, better off than marine animals? Possibly they are. In the confined and isolated fresh waters at least, the struggle for existence is undoubtedly less severe than in the waters of the sea. This is shown by the slow rate of change in freshwater types. Many of our genera of land and freshwater shells date back at least as far as to Purbeck and Wealden times, while our common pond-mussel is represented in the Coal Measures. The comparative security of fresh waters is probably the reason why so many marine fishes enter rivers to spawn.
More important, and less open to question, is the direct action of the sphere of life. The cheap method of turning the embryo out to shift for itself can seldom be practised with success on land. But in water floating is easy, and swimming not difficult. A very slightly-built larva can move about by means of cilia, and a whole brood can disperse far and wide in search of food, while still in a mere planula condition—hollow sacs, without mouth, nerves, or sense-organs. Afterwards the little locomotive larva settles down, opens a mouth, and begins to feed. Nearly the whole of its development is carried on at its own charge.
The extra risks to which marine animals are exposed also tell in favour of transformation, for they are met by an increase in the number of ova. Marine species commonly lay more eggs than freshwater animals of like habits. The Cod is said to produce nine million eggs; the Salmon from twenty to thirty thousand; the Stickleback only about one hundred, which are guarded during hatching by the male. The Siluroid fish, Arius, lays a very few eggs, as big as small cherries, which the male carries about in his mouth.
Without laying stress upon such figures as these, which cannot be impartially selected, we can safely affirm that marine forms are commonly far more prolific than their freshwater allies. But high numbers increase the difficulty of providing yolk for each, and thus tend to early exclusion, and subsequent transformation. We may rationally connect marine habitat with small eggs, poorly supplied with yolk, segmenting regularly, and producing larva which develop with metamorphosis.
In fresh waters dispersal can seldom be very effective. The area is usually small, and communicates with other freshwater basins only through the sea. Migration to a considerable distance is usually impossible, and migration to a trifling distance use less. Moreover, competition is not too severe to prevent some accumulation of food by the parent on behalf of the family.
On land the conditions are still less favourable to larval transformation. Very early migration is altogether impossible. Any kind of locomotion by land implies muscles of complicated arrangement, and, as a rule, there must be some sort of skeleton to support the weight of the body. The larva, if turned out in a Gastræa condition would simply perish without a struggle.[188] Nor is great precocity needful. The terrestrial animal is commonly of complicated structure, active, and well furnished with means of information. It can lay-by for its offspring, and nourish them within its own body, or at least by food stored up in the egg.
The influence of habitat upon development may be recapitulated as follows:—
Marine Habitat.—Eggs many. Yolk small. Segmentation often regular. Young hatched early. Development with metamorphosis. [The most conspicuous exceptions are Cephalopoda and marine Vertebrata.]
Fluviatile Habitat.—Eggs fewer. Yolk larger. Segmentation often unequal. Young hatched later. Development direct, or with late metamorphosis only. [The most obvious exceptions are Frogs and Toads, which develop with metamorphosis.]