26 NATURAL SCIENCE [July 



ture in different regions of the ocean. This map shows that the 

 surface of the sea may be grouped into five great zones, viz. : — (1) 

 A nearly continuous equatorial zone, where the temperature is high 

 and the range throughout the year does not exceed 10° Fahrenheit. 

 This zone includes all the principal coral-reef regions. (2 and 3) 

 Two polar zones, where the temperature is low and the annual range 

 likewise does not exceed 10° F. In these zones there are relatively 

 few lime-secreting organisms. (4 and 5) Two regions lying between 

 the equatorial zone and the two polar zones, where a wide range of 

 temperature occurs between the different seasons (the annual range 

 amounting to as much as 52° F. in some places). In these tem- 

 perate regions the secretion of carbonate of lime appears to be much 

 more active in the warmer than in the colder months. It thus 

 appears that the most favourable conditions for lime-secreting 

 organisms are met with in the warm, equable tropical waters of the 

 ocean, and here as a matter of fact we find the greatest develop- 

 ment of corals, and the largest number of lime-secreting pelagic 

 organisms. In the polar areas and in the cold water of the deep 

 sea there is, as is well known, a feeble development of all carbonate 

 of lime structures in marine organisms. 



From experiments which have been carried out by Mr Irvine 

 and myself at the Granton Marine Station, we have reason to believe 

 that this distribution is dependent primarily on the physical or tem- 

 perature conditions of the oceanic waters. When carbonate of lime 

 is precipitated by alkaline solutions, such as carbonate of soda, 

 carbonate of ammonia, or carbonate of methylamine, the effect of 

 temperature is very marked, and it appears to be the case that the 

 secretion of carbonate of lime by organisms is of the nature of a fine 

 precipitation in the interior of the soft structures. 1 If we add 

 sufficient carbonate of ammonia to sea-water at different temperatures 

 to convert all the lime salts present into carbonate, we obtain a pre- 

 cipitate which varies both in its crystalline form, in amount, and in 

 time of formation. At 32° F. the precipitate begins to form in 

 about six hours as small but distinct crystals of calcite, the quan- 

 tity in twenty hours amounting only to 0'2 gramme from a litre of 

 water. At a temperature of about 47° F. a mixture of calcite and 

 aragonite is precipitated; at 80° to 90° F. the quantity precipitated 

 is about 0*6 gramme, the precipitate begins to form in from a half 

 to one hour, and it appears to consist of minute crystals of aragonite. 

 It thus seems evident that carbonate of lime would be more easily 

 and more rapidly secreted in the high temperatures of the tropics 

 by means of the effete products of the organism. 



As is well known, carbonate of lime in any form is easily soluble 

 in water containing carbonic acid, and the aragonite form is more 



1 Murray and Irvine, I'roc. Roy. Soc. Ed/in., vol. xvii., pp. 79-109. 1S90. 



