FOSSIL INVERTEBRATED ANIMALS. 501 



instance of their having obtained, by sounding, some small 

 branching madrepores at the depth of twenty-four fathoms, in 

 56° south latitude ; and we know that, in the Mediterranean, 

 even the coral exists at a depth of ten or twelve hundred feet. 

 Now, as this is the case, may it not also happen that the 

 astraese live at a greater depth than these gentlemen have 

 stated, although they did not happen to find them so ? for 

 there is the greatest analogy between this genus of animals 

 and certain of the madrepores. Moreover, may not these reefs 

 and islands, which must certainly have had for their basis 

 some portion or projection of primitive, secondary, tertiary, 

 or volcanic stratum, which constitutes the bottom of the sea, 

 be at first increased to a certain height, by the assistance of 

 the numerous ramifications of the branching polypi, united 

 and solidified by the shelly animals which seek out these 

 anfractuosities, and the rest be then formed by the layers of 

 astraean and other encrusting polypi, whose action must be so 

 much the more lively and rapid, as the animals become sub- 

 jected to the more favourable influences of light and heat ? If 

 there be any probability in this conjecture, which has been 

 made by M. de Blainville, and we think that there is a great 

 deal, we may adopt a middle theory between that of Forster 

 and Peron, with whom the polyparia are the principal cause of 

 the growth and formation of the South Sea Islands, and that 

 of MM. Quoy and Gaimard,who will attribute to them only a 

 slight incrustation of a few feet. As to the support which the 

 opinion of Forster and Peron may derive from the fact of ma- 

 drepores being found on certain islands at very considerable 

 elevations, we must be careful to ascertain if the nature of 

 these islands be not volcanic ; for, in that case, these madre- 

 pores might have reposed upon the soil at very great depths, 

 and have been raised only with the volcanic substance itself, 

 or with any other which may have been thrown up by the 

 eruption. 



It is not very long since it has been asserted that we have 



