Chapter Nine
In which Darwin considers the absence of intermediate varieties and explains why our palaeontological collections are full of gaps
Chapter Ten
In which he describes how his theory can account for the pattern of succession from fossils to living forms
The geological record is like having just a few lines of a few pages of the history of the world. Much of it is incomplete, and because soft-bodied creatures are rarely preserved in the rocks some parts may be fragmentary at best. Since Darwin鈥檚 day, however, vast numbers of fossils have been found across the world, and his concern that the geological record fails to support his theory now seems unduly pessimistic.
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Darwin could only guess at the age of each stratum by estimating the rate at which rock was worn away. Today we can date fossils directly by examining how their chemical elements break down with time. The first life has been traced back more than 3 billion years and the death of the dinosaurs to 65 million years ago. Some of the records of that immense period are impressively complete.
The Himalayas are full of fossils 鈥� not of mountain creatures but of those of the sea, for long ago their peaks made up the floor of the Tethys Ocean. The fossils include the antecedents of great whales. The bones of the earliest ancestor of all cetaceans are found in beds some 50 million years old. They belong to a creature that had four legs and a tail, lived on the shore, and looked a little like a seal. Its ears, though, had a unique structure, now found only in whales.
The next prominent player, a million or so years later, 鈥渢he swimming-walking whale鈥� in Latin, looked like a 3-metre-long otter. Another million years on and the animal鈥檚 nostrils had migrated up the snout and the pelvis moved away from the backbone. A further 5 million years saw the oceans inhabited by a long mammal with tiny limbs.
Then came a great split. The ancestors of the blue whale and its relatives 鈥� those that filter tiny creatures from the water 鈥� began to develop gigantic sieves within their mouths, while others retained the sharp teeth found in earlier whales and in today鈥檚 killers. More recent deposits reveal the splits between dolphins and whales. A record that was once little more than an enormous gap has, with infinite labour and some luck, provided a complete history of the evolution of the largest animals that ever lived.
It has also given whales their rightful place 鈥� until not long ago quite obscure 鈥� within the family of mammals. Their earliest ancestors were close to those that gave rise to the hippopotamus. Whales, unique as they may seem, are hence members of a larger group that contains hippos, pigs, giraffes and cattle. DNA analysis backs up the record of the rocks. The whale鈥檚 whole story has been revealed within less than half a century.
Read more: On the Origin of Species, Revisited