« ПредыдущаяПродолжить »
Lion and Leopard, are copied so to speak by the Puma and Jaguar of tropical America ; the Tapir of Mexico mimics the congeneric animal of Sumatra ; the Sloth, Armadillo, and Myrmecophaga of Brazil, find relatives in the Manis and Orycteropus of Asia and Africa.
These parallels might be extended by many examples from Struthious and other birds, and from Crocodiles and other reptiles, tending to shew in two large separated regions, two distinct but analogous groups of life, subject to similar limitations by climate.
In like manner, reef-making Corals in the sea, and the large molluscous families of Cones, Cowries, and Volutes, might be mentioned as characterizing the warmer waters; but a more curious and interesting law of distribution of marine life, is founded on investigation of the contents of the sea at different depths.
INFLUENCE OF DEPTH ON THE DISTRIBUTION OF LIFE
IN THE SEA.
One example, the best known, is the Survey of the Ægean sea-depths by the late excellent naturalist, Edward Forbes. Dividing the depths from the surface to 230 fathoms into eight unequal zones, he finds
The first column gives the zones in succession downwards.
The second gives the limits of the zone in depth by fathoms.
The third, the depth in thickness of each zone in fathoms.
The fourth the number of species found in each
The fifth is a column which I have calculated by dividing the fourth by the third, to shew the relative productiveness of each zone. (For the sake of avoiding decimals, the quotient is multiplied by 10.)
Thus it appears that the relative fertility of the several zones decreases downwards toward zero : and it is believed that at 300 fathoms life is extinct.
If we consider that at and near the surface of the sea all the influences favourable to both vegetable