Bret: Darwin... His commute in the field, at least sometimes, involved riding tortoises.
Heather: Yes, so you said this to me earlier today, and I was like, I gotta find this. So, apparently, it's in the Voyage on the Beagle, we have it, but I'm like, I can't find it in here. I found references in peer-reviewed papers that suggest that yes that's true, so... I can't find a quote from him saying "today I rode a tortoise"
Took about half an hour to locate the relevant passage. From the second edition of the Voyage, published here. Second paragraph, third sentence: