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! 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, Proc. Roy. Soc. Edin., vol. xvii., pp. 79-109. 1890. 
