. Animal life as affected by the natural conditions of existence. Animal ecology. paljEontological argument. 137 an inconceivable variety of these forms was brought to light by the ' Challenger ' expedition. A woodcut is here given of one of the most beautiful of these species, the Euplectella, which belongs to the group of Sponges (fig. 34). Although the results of the ' Challenger' expedition have not yet been fully published, so that it is impossible to give a complete list of the various deep- sea forms and their distribution vertically in depth, it seems to be tolerably certain that they
. Animal life as affected by the natural conditions of existence. Animal ecology. paljEontological argument. 137 an inconceivable variety of these forms was brought to light by the ' Challenger ' expedition. A woodcut is here given of one of the most beautiful of these species, the Euplectella, which belongs to the group of Sponges (fig. 34). Although the results of the ' Challenger' expedition have not yet been fully published, so that it is impossible to give a complete list of the various deep- sea forms and their distribution vertically in depth, it seems to be tolerably certain that they have a much wider vertical distri- bution in tropical seas than in northern oceans; in the north, for instance, no Euplectella nor any allied form of sponge—six- rayed Siliceous Sponges—has been found in a less depth than, 300 fathoms, whUe in the Indian Ocean they are common in 100 fathoms or less. Thus the higher temperature of the water to which these cold-water animals*^ are exposed in tropical. Fig. H.—Euplectella Aspergillum, a siliceous sponge of a group -whicll consists mostly of fossil forms. seas is in no way prejudicial to their existence; and this can only result from the fact that these animals are better able to bear a difference of temperature, so long as it remains equable, than variations between two extremes lying far apart, and to which they are more or less suddenly exposed. Here, in conclusion, we must briefly discuss an application of the foregoing statements and arguments to Palaeontology. It is generally assumed that we are justified in attributing to extinct animals a mode of life analogous to that of the nearest related surviving forms. But, in the first place, it Ls often extremely difiicult to decide what may have been the nature of the affinities between extinct and living animals, and it cannot be disputed that, in instituting such comparisons, we are often obliged to judge by characters which in no way warrant our. Please note that these ima
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