My life : a record of events and opinions - Wallace-online.org

384 MY LIFEof brilliant male coloration or marking being due tofemale choice.6. The general permanence of oceanic and continentalareas was first taught by Professor J. D.Dana, the eminent American geologist, and again byDarwin in his " Origin of Species "; but I am, Ibelieve, the only writer who has brought forwarda number of other considerations, geographical andphysical, which, with those of previous writers,establish the proposition on almost incontrovertiblegrounds. My exposition of the subject is given in"Island Life" (chap, vi.), while some additionalarguments are given in my " Studies " (vol. i. chap. ii.).The doctrine may be considered as the only solidbasis forany general study of the geographical distributionof animals, and it is for this reason that Ihave made it the subject of my careful consideration.7. In discussing the causes of glacial epochs Ihave adopted the general views of Mr. James Croll asto the astronomical causes, but have combined themwith geographical changes, and have shown how thelatter, even though small in amount, might producevery important results. In particular I have laidstress on the properties of air and water in equalizingtemperature over the earth, while snow and ice, bytheir immobility, produce cumulative effects ; and thusa lowering of temperature of a few degrees may leadto a country being ice-clad which before was ice-free.This is a vital point which is the very essence ofthe problem of glaciation ;yet it has been altogetherneglected in the various mathematical or physicaltheories which have recently been put forward.Myown discussion of the problem in chapter viii. of