PREFA TOR Y NOTE. ix 



begun in so deductive a spirit as this, for the whole theory was 

 thought out on the west coast of South America, before I had 

 seen a true coral-reef. I had only to verify and extend my views 

 by a careful examination of living reefs. But it should be 

 observed that I had during the two previous years been inces- 

 santly attending to the effects on the shores of South America of 

 the intermittent elevation of the land, together with denudation 

 and the deposition of sediment. This necessarily led me to 

 reflect much on the effects of subsidence, and it was easy to 

 replace in imagination the continued deposition of sediment by 

 the upward growth of corals. To do this was to form my theory 

 of the formation of barrier-reefs and atolls.' Coupled with this, 

 Darwin, as is manifested by his work, also saw that, in forming 

 any theory of the genesis and development of coral-reefs, not 

 only must the nature of the platform on which the corals build 

 be taken into account, but that other factors, of as equally great 

 importance, come into play and must be reckoned, — notably, 

 the peculiar conditions of the life of the coral-polypes themselves, 

 and the peculiar, and, then, inexplicable distribution of the reefs 

 and atolls. He saw more clearly than his precursors had 

 done the validity of the dictum of Johannes Miiller, in this and 

 indeed in all his works, that the most important truths in 

 Natural Science are to be discovered, neither by the mere 

 analysis of philosophical ideas, nor by simple experience, but by 

 reflective experience, which distinguishes the essential from the 

 accidental in the phenomena observed, and thus finds principles 

 from which many experiences can be derived. 



The conditions necessary for the proper continuance of the 

 organic life of these polypes seem to be a temperature which is 

 not lower than 68°F., the presence of clear water, and a depth not 

 exceeding twenty fathoms. They also cannot survive exposure 

 to the sun and air, and thus are unable to flourish unless the top 

 of the reef be below the mark of the lowest tides. Thus reefs 

 are absent from the West Coast of America because it is washed 

 by a cold extra-tropical current ; they are not found in the 

 South Atlantic because that ocean is not within their special 

 isotherm; and they are not present on the shores of Trinidad 



b 



