360 DISCOVERY REPORTS 



organisms in warmer waters is another important biological character distinguishing 



the tropical from the polar regions. 



As we pass from the polar regions through the temperate to the sub-tropical and 

 tropical seas we pass, broadly speaking, from the regions of high nutritive salt content and 

 rich " free ' ' phytoplankton with apparent exclusion effects to regions of very low nutritive 

 salts content and "imprisoned" (symbiotic) phytoplankton without the effects of the 

 exclusion of the animal plankton from the sunlit surface layers. It may be significant 

 that radiolarian ooze is found only under stretches of the tropical Pacific Ocean and 

 part of the Indian Ocean, but not on the floor of the Atlantic Ocean, suggesting that the 

 Radiolaria may be more prolific and fill an ecological gap in the water which has travelled 

 a longer distance from the temperate regions of greater free phytoplankton production, 

 the distances travelled by such water in the Atlantic Ocean being very much less. The 

 sequence of deep-sea deposits encountered as one passes from the polar regions to 

 the tropics in the wide Pacific Ocean, i.e. diatom ooze, globigerina ooze and radiolarian 

 ooze, may reflect a fundamental change in the condition of the surface water as it is 

 carried over great distances away from regions of predominantly "free" phytoplankton 

 to regions of predominantly "imprisoned" phytoplankton. Darwin was dissatisfied 

 with temperature as a factor governing the distribution of coral reefs ; although highly 

 speculative it is perhaps worth keeping in mind the possibility that their distribution 

 may be associated with water masses which by currents have travelled farthest from the 

 predominantly "free" phytoplankton regions, or perhaps more reasonably that they 

 occur in water in which the "free" phytoplankton never or only rarely reaches a con- 

 centration sufficiently high to bring about exclusion effects. Marshall (1933) has recently 

 shown that the "free" phytoplankton production of the Great Barrier Reef region is 

 very low. Orr (1933), in describing the physical and chemical conditions in the waters 

 of the Great Barrier Reef region, writes: "There is apparently seldom a complete lack 

 of either of these salts (phosphates and nitrates), so that plant life is always present ; 

 the quantities, however, are too small to allow of a striking outburst of growth, such as is 

 characteristic of temperate seas". Coral reefs are not found on the south-western 

 coasts of Australia or the west coasts of North and South America, or on the eastern 

 islands of the Pacific, and, like the radiolarian ooze, typical coral reefs are not found in 

 the smaller Atlantic Ocean, and those that are found there are bathed by water which has 

 travelled farthest from the temperate regions in the north and south equatorial currents. 1 

 It may be that there are relevant conditions in the water, chemical or ecological, 

 which are only indirectly linked with the reduction in phytoplankton density. 



These suggestions are only put forward in the hope that the critical consideration and 

 likely rejection of new hypotheses may lead to fresh observations and experiments, 

 which will ultimately furnish us with a better understanding of the ecological relation- 

 ships between pelagic animals and plants. 



1 Since this was written I find that Potts (1923) associated the distribution of coral reefs in the Pacific 

 with ocean currents and points out the progressive impoverishment in the number of coral species met 

 with as the ocean is crossed from west to east. He regards, however, the influence of the currents to 

 be one of temperature. 



