66 ON CORAL REEFS AND ISLANDS. 



era in most instances, and moreover there are no abrupt transi- 

 tions ; consequently the resulting reefs should have a nearly uni- 

 form character, as here stated. This fact may be better appreci- 

 ated after perusing the following chapter. 



The Nullipore zone along the reef-line between low and high 

 tide, is clearly made out by Mr. Darwin, and is one of the inter- 

 esting results of his investigations. It performs a very important 

 part, by the protection it gives the reef from abrasion. The expo- 

 sed reef is thus gathering lime from the waters, and extending 

 itself, when, if devoid of this protection, it would be constantly 

 yielding to the sea. Ou the inner reefs where the protection is 

 not needed, it is not given ; some species of Nullipore, however, 

 occur in these regions, and others are found at various depths. 



As the Caryophyllia family extend into deeper waters than 

 most other reef-corals, it might be inferred that these at least may 

 constitute a lower bed, or substratum. But this is by no means 

 the case. As just stated, one species (the Dendrophyllia nigres- 

 ceus) was found at 14 \ fathoms and also of identical characters 

 at low tide level. The Caryophylliae are but sparingly distributed ; 

 the species are few, and mostly small ; and not a dozen different 

 kinds were detected in the Pacific. Their contributions to reefs 

 are therefore inconsiderable. 



4. Rate of Growth of Zoophytes. 



The rate of growth of zoophytes, is a subject but little under- 

 stood. We do not refer here to the progress of a reef in forma- 

 tion, which is another question complicated by many co-operat- 

 ing causes; but simply to the rapidity with which particular 

 species of coral-zoophytes increase in size. There is no doubt 

 that the rate is different for different species. It is moreover 

 probable that it corresponds with the rate of growth of other 

 allied polyps that do not secrete lime. The rate of growth of 

 Actiniae might give us an approximation to the rate of growth in 

 a Mussa, which are coral animals of like size and general char- 

 acters ; for the additional function of secreting lime, would not 

 retard necessarily the maturing of the polyp ; and from the rate 

 of growth of the same animals in the young state, we might per- 

 haps draw some inferences as to the rate in polyps of correspond- 

 ing size. But no observations on this point were made by us 

 while abroad. 



Although the rapidity is undoubtedly far less than was for- 

 merly reported, the following facts from different sources seem to 

 show that the rate is still greater than has been of late believed. 

 Mr. Darwin, citing from a manuscript by Dr. Allen of Forres, 

 some experiments made on the east coast of Madagascar, states 

 that, in December, 1830, twenty corals were placed by this gen- 

 tleman apart on a sandbank, in three feet water, (low tide,) and 



