“I wonder if it would be difficult to ride on such a shell,” I said idly, then kicked myself. That wasn’t the kind of thing Mr. Darwin wanted to hear from his assistant!

“Well, ready yourself, then,” said Mr. Darwin, and to my surprise he clambered aboard one of the giant tortoises, perching on top of its shell. “What are you waiting for?”

The ancient animal stretched out its long crusty neck and hissed at the unexpected weight, then took a ponderous step. Mr. Darwin just managed to catch his balance. His laugh rang out, much clearer and louder than his voice, and he slapped his thigh.

This was more like it. The master might be awful clever and mostly serious, but he was only a young man himself, and I loved those rare moments he was game for a laugh. We’d been measuring tortoise shells all day and a break was more than welcome.

I eyed up the tortoises and chose a smaller one that seemed to be fast asleep, its head tucked into its wrinkled neck like an acorn in its cap. I scrambled onto its back. It wasn’t as easy as Mr. Darwin made it look. My master was tall and sometimes stooped. He had a way of swinging his arms when he walked but wasn’t nearly as clumsy as he looked. My knees slipped on the mottled shell, but I finally managed to settle my behind in the center. When the tortoise started to move, I felt as though I was back on the Beagle sailing around the stormy waters of Cape Horn.

Mr. Darwin’s tortoise was heading across the lava field, but mine stopped and dipped to munch some grass, nearly tipping me off.

“You’ve chosen a donkey, Covington, but mine is a noble steed!” called Mr. Darwin.

I laughed out loud as he waved his hat in the air. If only Da could see me now.

A shadow darted over me and I looked up. Two magnificent frigate birds swooped on the air currents, massive black wings almost as sharply pointed as their beaks and tails. Their red throats flashed.

Mr. Darwin stared upward too. “Looks like some weather coming in, Covington …”

I could see it too. The sky was suddenly the color of a bruise and the air smelled of copper pennies.

The young sir hopped down from the tortoise. “Did you pack the specimens well?” His voice was serious once more.

“I did, sir,” I replied, and slithered down myself. My tortoise had tucked its head back in. I pulled up some grass, which it took from my offered hand with a beaky, toothless mouth. I liked the tortoises; there seemed to be a lot of thinking going on behind those old black eyes.

“Make haste, then, boy. Let’s get them back to the barrels,” said Mr. Darwin.

The casks of wine would preserve the specimens we’d collected until we landed at a port where they would be sent all the way back to Mr. Darwin’s colleagues in Cambridge.