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i. 



^ 



' On Coral Reefs and Islands. 179 



The effects of sediment on growing zoophytes are strongly 

 markedj and may be often perceiVed when a minghng of fresh 

 water alone produces little influence. We have mentioned that 

 the Porites are reduced to flattened masses by the lodgment of 

 sediment. Tl\e same takes place with the hemispheres of As- 

 traea; and it is not uncommon that in this way large areas at top 

 are deprived of life. The- other portions still live unaffected by 

 the injury thus sustained. Even the Fungias, which are broad 

 simple species, are occasionally destroyed over a part of the disk 

 through the same cause, and yet the rest remains ahve. Wher- 

 ever streams or currents are moving or transporting sediment, 

 there no corals grow ; and for the same reason we find no living 

 zoophytes upon sandy or muddy shores. 



The influence of temperature on the development of animal 

 lifcj and the distribution of species is well known. But in no 

 department is it more strikingly displayed than in that of zoo- 

 phytes. In a former report we have considered the general in- 

 fluence of temperature on the several divisions of this order of 

 animals. The remarks which follow are consequently confined 

 to the reef-forming species. We reserve for still another page 

 the influence of this cause on the distribution of reefs, since we 

 are occupied here with zoophytes as animal species, and not with 

 reefs, a result from the growth of corals. 



The temperature of the ocean in which reef-corals grow is 

 evidently the temperature congenial to them. From a general 

 survey of facts, it appears that these species are not met with 

 where the winter temperature remains much time below 66° F., 

 though a temporary reduction to 64°, or perhaps lower, (as the Ber- 

 mudas,) may sometimes occur. Where the temperature is above 

 this, even in the hottest parts of the torrid zone, coral zoophytes 

 thrive well. An isothermal line, crossing the ocean where this win- 

 ter-temperature of the sea is experienced, one north of the equator, 

 and another south, bending in its course by divergence or converg- 

 ence, wherever the marine currents change its position, will in- 

 clude all the growing reefs of the world ; and the area o^ waters 

 ^ay be properly called the coral-reef seas. This limiting tem- 

 perature is found near latitude 28°. Under the equator in the 

 Pacific, the waters where warmest, have the temperature 85° F., 

 and in the Atlantic 83° F. ; 66° to 85° is therefore not too great 

 a range of temperature for the various reef-forming corals. Par- 

 ticular species, however, have smaller limits; but these limits 

 have not yet been accurately ascertained.* 



Tlxe first application of the well-established principle that temperature influ- 

 ences the growth and tlibtributiou of corals is claimed by lit. J. P. Coufnouy equally 

 With myself. Anv attempt, however, to determine a limiting temperature he dis- 

 Claims, and in this particular, as well as the conductions anived at, our views are 

 ^t;ry different. Tlie facts and inferences stated in this place, and on a following 

 page, are UwJuced throughout from my own study and investigation. 



