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 



