. Island life; or, the phenomena and causes of insular faunas and floras, including a revision and attempted solution of the problem of geological climates. argely, though very slowly,and it is now nearly at a minimum. It also varies veryirregularly; but its amount has been calculated for severalmillion years back. Fifty thousand years ago it was ratherless than it is now, but it then increased, and when we cometo a hundred thousand years ago there is a difference of 124 ISLAND LIFE. [rART I. eight and a half millions of miles between our distance from thesun in aphelion and perehelion (as the
. Island life; or, the phenomena and causes of insular faunas and floras, including a revision and attempted solution of the problem of geological climates. argely, though very slowly,and it is now nearly at a minimum. It also varies veryirregularly; but its amount has been calculated for severalmillion years back. Fifty thousand years ago it was ratherless than it is now, but it then increased, and when we cometo a hundred thousand years ago there is a difference of 124 ISLAND LIFE. [rART I. eight and a half millions of miles between our distance from thesun in aphelion and perehelion (as the most distant and nearestpoints of the earths orbit are termed). At a hundred and fiftythousand years back it had decreased somewhat—to six millionsof miles; but then it increased again, till at two hundredthousand years ago it was ten and a quarter, and at two hundredand ten thousand years ten and a half millions of miles. Byreference to the accompanying diagram, which includes thelast great period of excentricity, we find, that for the immenseperiod of a hundred and sixty thousand years (commencing jCPROBABLE DURATION OF THE CLACIAt CPOCH>>. 100 ~ 50 0 THOUSAND YEARS AGO DIAGRAM OF EXCENTRICITY AND PRECESSION. The dark and li<,ht bands mark the phases of precession, the dark showing short mild winters,and the lidit long cold winters, the contrast being greater as the excentricity is horizontal dotted line marks the present excentricity. The figures show the maximaand minima of excentricity during the last 300,000 years from Dr. Crolls Tables. about eighty thousand years ago) the excentricity was verygreat, reaching a maximum of three and a half times itspresent amount at almost the remotest part of this period, atwhich time the length of summer in one hemisphere and ofwinter in the other would be nearly twenty-eight days in , during all this time, our position would change, as abovedescribed (and as indicated on the diagram), ever
Size: 2361px × 1059px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorwallacealfredrussel18, bookcentury1800, bookdecade1880
