XVI INTRODUCTION. 



tata.) This is the case with all and their peculiar 

 habitats will be noticed when speaking* of each 

 species. This selection of situations, however 

 is a physiological one, rather than one which 

 may be supposed to be the result of any intel- 

 ligence in the animals themselves. For in the 

 same situations of climate and depth of water, 

 the same species will frequently grow on any 

 substances provided there are similarities of 

 surface. This is however considerably modified 

 by collateral circumstances, such as the power 

 to absorb and radiate heat and the durability of 

 the substances on which they grow. Thus for 

 instance the Caryophyllia Smilhii most commonly 

 selects for itself the moderately rough surfaces 

 of stones, yet it has been repeatedly found in a 

 young state on the lobes of Alcyonium digit alum, 

 and on the stems of fuci, but in such situations 

 it never attains a full size. The young of all the 

 species will take root and grow for a short time, 

 in situations which afterwards appear to be des- 

 tructive to them. Those ova therefore which 

 fix themselves in unfavourable situations perish, 

 while others more favourably situated thrive and 

 grow to perfection ; but still it remains a matter 

 for speculation, why one kind of locality should 

 be more favourable to one species than to an- 

 other, since they derive no nutriment from the 

 root. 



It is a generally received opinion, that Zoo- 

 phytes do not inhabit the depths of the ocean, 

 but are confined to shallow water and to districts 

 surrounding the shores. The depths to which 

 the coral reefs descend in the Lagoon islands 

 is not considered a contradiction to this opinion, 

 since they are supposed to owe their position to 

 subsidences, in the same manner that the corals 

 in the hills of those islands are placed there by 



