He said so himself. And he regretted it.
Exhibit A, from The Autobiography of Charles Darwin:
“[Not being urged to practice dissection] has been an irremediable evil, as well as my incapacity to draw.”
It was actually Darwin’s shipmate on the HMS Beagle, Conrad Martens, who made the sketches best known from that expedition. And, it wasn’t until well after Darwin’s famous voyage to the Galapagos that a publisher sent an artist back to that region with the express responsibility to illustrate Darwin’s observations.
Most publications from Darwin’s era were similarly professionally illustrated, with many of the illustrations based on specimens he collected. However, these illustrations were not Darwin’s own work.
Mind you, Darwin did occasionally sketch, as can be seen in his diagrams of “trees” roughly indicating how organisms were related. And, there are a handful of rough sketches of plant cross sections and geologic formations scattered through his myriad notebooks. But, these few sketches pale alongside the copious volumes of written notes and manuscripts he made.
Darwin maintained he couldn’t draw.
So he didn’t ever do it.
His colleagues are said to have bemoaned his lack of skills and reluctance to practice. Despite encouragement from luminary scientists of the day. Despite the utility of his rare sketches (which are absolutely functional in terms of conveying information). Despite drawing being standard training for scientists in his era. Despite all that, Darwin restrained from using drawing as a regular tool in his work.
Darwin’s dilemma is a cautionary tale.
Imagine what we’d be able to “see” through his eyes if he had drawn more often. Seeing firsthand what someone else discovered or found compelling is really hard to do if we’re limited to written descriptions. There is only so much one’s word choice can do to conjure the same image in someone else’s mind.
Only a couple of years ago, some 57 pages of the Darwin children’s doodles were found…on the back of the original manuscript for The Origin of Species! Those are the extant pages of what was thought to be a missing manuscript seminal to the history of science.
Drawing became an essential part of science way before Darwin, and it’s one aspect of his legacy where we see that he “missed the boat.” Evidently, Darwin himself wouldn’t think I’m taking too much liberty by saying so. He encouraged his children (three of whom grew up to be a botanist, astronomer, and engineer) in the skill he himself lacked.
 This reluctance to draw is common among all ages. In the workshops and educator trainings I facilitate, I find such reluctance is often due to a combination of social conditioning (risk aversion and notions of other people having “innate talent”) and limited-to-no training and practice in drawing techniques. See previous posts and newsletters for a lot of resources (sketching tips, a sketching guide, recommended readings, etc.) which can help you and/or your students overcome these concerns. It’s not that Darwin physically couldn’t draw. It’s more likely (considering how he writes about it) that he was reluctant to commit the time and effort necessary to become proficient at it. And, that reluctance is a constraint we can overcome. In particular, see my Suggestions for Integrating Drawing into University-Level Biology Courses for lots of research and recommendations for overcoming mindset and skill-based concerns about drawing. These resources are relevant even if you do not teach. And, based on recent feedback from k12 teachers I worked with during this summer Teaching Institute, the material is also helpful for educators of younger students.