Fossils raised many questions about the origin of species--and not just for Darwin. Discoveries in geology had already challenged the idea that the world and all its species had been created at the same time a few thousand years ago. Fossils clearly showed that in past ages, the world had been inhabited by different species from those existing today. Old species had died out, and new species had appeared at many different times in Earth's history.

Fossils also revealed another intriguing pattern: New species tended to appear where similar species had previously lived. Why would one species replace a similar one in the same location? Or perhaps, Darwin would eventually wonder, had the older species somehow given rise to the new ones? Back in London, the relationship between old and new species, as shown in fossils, would become one of the main lines of evidence leading to Darwin's theory of evolution.