Darwin · On the Origin of Species
Why is life so diverse?
That time, that place
Five years aboard the Beagle. In the Galapagos he saw that finch beaks and tortoise shells differed slightly from island to island. He did not grasp the meaning then, and did not even label his specimens carefully by island.
Back home he published nothing for over twenty years. He spent eight of them on barnacles, and bred pigeons himself.
Why this question
Two things were needed: evidence that species change, and a mechanism for how. Lamarck had the first and got the second wrong.
The clue came from an unlikely place. Reading Malthus on population, he caught on the fact that far more are born than can survive.
What was found
The argument takes three lines.
Individuals within a species differ slightly. Some of those differences pass to offspring. And far more are born than can survive.
If all three hold, the conclusion follows: whatever difference gives even a slight edge leaves more descendants, and repeated long enough, species divide.
No designer is required. There is no purpose and no direction — only difference, time, and death.
Something was missing. He did not know why variation arises or how traits are transmitted. That answer was being worked out that very year in a monastery in Brno, and the two never met.
Species were as created, and intricacy was evidence of design
Changes breeders produced in a few generations, and species split island by island
When Wallace reached the same conclusion and wrote to him, twenty years of delay ended in a rush