Charles Darwin had all the anxieties typical of a recent college graduate: uncertain of what was to come, depressed by the prospects available in the Real World. He had already disappointed his physician father once by dropping out of medical school, so he studied theology at Cambridge in order to take up the gentlemanly profession of being a parson. But young Charles was clearly procrastinating and desperately planning one last hurrah of the naturalist’s life. Then one late summer’s day, he opened a letter offering him an opportunity to sail around the world aboard the HMS Beagle. . . .
28 January 1832, St. Jago
Found amongst the rocks West of Quail Island at low water an Octopus.— When first discovered he was in a hole & it was difficult to perceive what it was.— As soon as I drove him from his den he shot with great rapidity across the pool of water.— leaving in his train a large quantity of the ink.— even then when in shallow place it was difficult to catch him, for he twisted his body with great ease between the stones & by his suckers stuck very fast to them.— When in the water the animal was of a brownish purple, but immediately when on the beach the colour changed to a yellowish green.— When I had the animal in a basin of salt water on board this fact was explained by its having the Chamælion like power of changing the colour of its body.— The general colour of animal was French grey with numerous spots of bright yellow. . . . Over the whole body there were continually passing clouds, varying in colour from a “hyacinth red” to a “Chesnut brown”.— As seen under a lens these clouds consisted of minute points apparently injected with a coloured fluid. The whole animal presented a most extraordinary mottled appearance, & much surprised very body who saw it. . . . The animal seemed susceptible to small shocks of galvanism: contracting itself & the parts between the point of contact of wires, became almost black.— this in a lesser degree followed from scratching the animal with a needle.— The cups were in double rows on the arms & coloured reddish.— The eye could be entirely closed by a circular eyelid.— the pupil was of a dark blue.— The animal was slightly phosphorescent at night.
30 January 1832, St. Jago
Found another. changed its colour in the same manner when first taken. Caught another: I first discovered him by his spouting water into my face when I certainly was 2 feet above him. When seen in water was of dark colour with rings: being with difficulty removed from a deep hole & placed in a puddle of water swam well & emitted a dark Chesnut brown ink.— he continued likewise to spout water, evidently being able to direct his siphon.— When on land did not walk well having difficulty in carrying its head which it continued filling with air as before with water.— From same cause the animal often made a noise when squirting out water. They are so strong & slippery that one hand is insufficient to hold them.— Whilst swimming generally changed colour & seemed to imitate colour of the rocks.—
Darwin was obviously mesmerized by the way the octopus changed its colors like “passing clouds” and he noted the “minute points apparently injected with a coloured fluid.” What he saw were chromatophores, specialized cephalopod pigment cells. Each chromatophore is individually innervated (labeled “Axon” in lower left of the diagram) and equipped with radial muscle fibers that pull the chromatophore from a dense ball of dark pigment into a flat, splayed out pigment field that blossoms with color.
Cloney, Richard A. & E. Florey. 1968. “Ultrastructure of Cephalopod Chromatophore Organs.” Zeitschrift für Zellforschung 98: 250 – 280.
The Complete Work of Charles Darwin Online. John van Wyhe, ed. 2002. http://darwin-online.org.uk/
Desmond, Adrian & J. Moore. 1991. Darwin: The Life of a Tormented Evolutionist. W.W. Norton & Company.