origin of the phrase 'survival of the fittest'

I'm reading a book about the discovery of the most complete early primate fossil ever found. It's called 'The Link' and it's by Colin Tudge. (If you click on the link - no pun intended - to the book, you'll arrive at a wealth of information about this fossil, and you can download the first chapter.)

I'm fascinated by the story of Ida, the young creature who breathed carbon dioxide, died and fell into a pool, her body preserved as a perfect specimen because there was no air at the bottom of the pool. But I didn't expect the laugh-out-loud moment that came with the reading of a paragraph about the phrase 'survival of the fittest'.

Tudge explains that in the middle of the nineteenth century the word fittest didn't have the dominant connotation it has now, 'of health and raw strength'. To the Victorians the word meant suitable or apt, and Charles Darwin was quite clear that natural selection does not need to result in creatures that are superior to their predecessors. Tudge explains:

He [Darwin] had spent a long time studying barnacles, after all, and barnacles are astonishingly successful - we find them everywhere. But they descended from free-living, shrimplike ancestors and became barnacles by losing their brains and sticking themselves head-first to rocks - hardly a great leap forward, but it worked.

I love this explanation!

By the way, Darwin didn't coin the expression survival of the fittest. It was first used by Herbert Spencer.