Third, Darwin argued that the geographical distribution of species supported
his theory. Why did most marsupials live in Australia? Why were there
no wolves in Australia, even though there was a “marsupial wolf,” the
Thylacinus, living in environments that would have suited wolves very well?
Darwin argued that this was because Australian species were descended from
local ancestors. Yet a deistic theory of life could offer no reason why a deity
should not have placed species such as wolves in all parts of the world for
which they were designed.
As a scrupulous scientist, Darwin was also painfully aware that there were
gaps in his evidence. However, most of the gaps have been ¿ lled in since
his death. The fossil record is now much richer. Even today, it is impossible
to produce a series of fossils showing all the links between one species and
another because so few organisms are ever fossilized. However, many more
“transitional” fossils have been found. Modern dating techniques also enable
us to date fossils precisely, putting them on timelines that demonstrate
clearly how species have changed over time. A particularly striking example
is the detailed reconstruction of the evolution of modern horses from a small,
fox-sized animal—Hyracotherium—that lived about 60 million years ago.
Darwin feared that contemporary estimates for the age of the Earth (20–400
million years, according to Lord Kelvin) allowed too little time for natural
selection to generate the many species present in the modern world. As we
have seen, modern estimates give an age of more than 4 billion years for the
Earth, providing plenty of time for natural selection to create new species.
Darwin and his contemporaries had little understanding of how heredity
works. Darwin assumed that qualities inherited from parents were normally
blended in their offspring. Unfortunately, this seemed to mean that unusual
but advantageous mutations, far from being preserved, would be diluted
generation by generation. And that suggested that species ought to remain
stable over time. Modern genetic studies began with the work of Gregor
Mendel (1822–1884), whose work was not widely known before the 20th
century. Mendel showed that many features, such as the colors of eyes, were
not blended. The discovery of DNA in 1953, and subsequent clari¿ cation
of how genes work, has demonstrated that heredity works in just the ways
required by Darwin’s theory. While some parental features are blended,
many are not. Furthermore, though the copying of genes is almost perfect,