CHAPTERS ELEVEN AND TWELVEGeographical DistributionIn which Darwin demonstrates the importunce a/geographicalbarriersandclimate change to exploin the distribution oflife as wesee ittodoy

Penguins are charmingbirds with 1500 kilometres. Alreadythe ancestralbetween 17 and 20 species, depending penguins had almost lost theirwings.onwhich classification is used. They Fossils and DNAeach show that thetake a variety offorms, ranging from penguins' spread matches the advance

. the statuesque emperorfound in the and retreat ofthe ice. Starting aroundAntarctic, to the fairy penguin of 35 million years ago, a series ofice agesAustralia, just t/20th its weight. Other made the Antarctic uninhabitable, andpenguins llve In New zealand, South the birds retreated northwards as theAfrica, SouthAmericaand the Galapagos. glacier spread. Inaddition, Icy currentsTheycannot lIy (although the DNA swept some westwards, eastwards andsuggests that albatrosses are among closer to the equator.theirclosest relatives), so how did they Afterto millionchillyyears, the worldreach such scattered places? warmed, cooled andwarmed again, and

The oldest penguinfossils appear the birds followed the retreating edge ofshortlyafter the demise ofthe dinosaurs the ice sheet into the far south. They ieftaround 65 million years ago. This behind colonies scattered on cold-waterancestor ofall living species lived in shores across the globe. In the yearssouthern New Zealand and Marie since then, isolation and the challengesByrd Land, Antarctica, which were at faced by each separate group have ledthat time separatedby less than to the diversityofpenguins seentoday.

CHAPTER NINEOn the Imperfection of the Geological RecordIn which Darwin considers the absence ofintermediate varieties andexplains whyourpalaeontolagical callemans arefull afgaps

CHAPTER TENOn the Geological Succession of Organic BeingsIn which he describeshowhis theorycan acrountforthepattern afsuccessianfromfossils to livingformsThe geological record Is llke having just others retained the sharp teethfound Ina few lines ofafew pages of the history earlierwhales and In today's kiliers.of the world. Much afit is incomplete, More recent deposits reveal theand because soft-bodied creatures are splits between dolphins and whales.rarely preserved in the rocks some parts Arecord thatwas once little moremaybe fragmentary at best. Since than an enormous gap has, with infiniteDarwin's day, however, vast numbers labour and some luck, provided aoffosslls have been found across the complete history ofthe evolution of theworld, and his concern that the largest animals that ever lived.geological record falls to support his It has also given whales their rightfultheory now seems unduly pessimistic. place-until not long ago quite obscure-

Darwin could onlyguess at the age within the family ofmammals. Theirofeach stratum byestimating the rate at earliest ancestors were close to thosewhich rock was worn away. Todaywe can that gave rise to the hippopotamus.date fossils directlyby examining how Whales, unique as they may seem, aretheirchemicalelementsbreakdownwith hence members ofa larger group thattime. The first life has been traced back contains hippos, pigs, giraffes and cattle.more than3 billionyears and the death DNAanalysis backs up the record oftheof the dinosaurs to 65 millionyears ago. rocks. The whale's whole storyhas beenSome of the records ofthat immense revealed within less than haifa century.period are impressivelycomplete.

The Himalayas are full offossils-notofmountain creatures but ofthose ofthe sea, for long ago their peaks made upthe lloor of the Tethys OCean. The fossilsinclude the antecedents ofgreat whales.Thebones ofthe earliest ancestorofall cetaceans are found inbeds some50 million years old. They belong toa creature that had four legs and a tall,lived on the shore, and looked alittlelike a seal. Its ears, though, had a uniquestructure, now found only in whales.

The next prominent player, a mIlIlonor so years later, "the swimmlng-wallclngwhale" in Latin, looked like a 3-metre­long otter. Another millionyears onand the animal's nostrils had migratedup the snout and the pelvis moved awayfrom thebackbone. Afurther 5 millionyears saw the oceans inhabited by along mammal with tiny limbs.

Then came agreat split. The ancestorsofthe blue whale and its relatives­those that filter tiny creatures fromthe water-began to develop giganticsieves within their mouths, while

Page six 1NewScientist 114 November 2009