stretch
Explanation
This comic is a history-of-science joke about Jean-Baptiste Lamarck, the French naturalist famous for his (now-discredited) theory of inheritance of acquired characteristics.
Lamarck's theory proposed that organisms could pass on traits they developed during their lifetime to their offspring. The classic example is that giraffes got long necks because their ancestors stretched to reach high leaves, and this stretching was inherited by the next generation. This was eventually superseded by Darwinian natural selection and Mendelian genetics.
In the comic, someone calls out to "Jean-Baptiste" asking what he's doing in there "for hours every night." His response from behind a closed door: "Stretching! Stretching for the benefit of future generations!"
The caption reads: "After proposing his theory of inheritance of acquired characteristics, Lamarck spent most of his evenings trying to lengthen his penis."
The joke is wonderfully crude and logical at the same time: if Lamarck truly believed that physical changes acquired during one's lifetime would be passed to offspring, then the most rational application of his own theory would be to "stretch" the body part he'd most want future generations to inherit in enhanced form. The humor comes from applying a scientific theory to its most embarrassing logical conclusion, combined with the image of a renowned scientist earnestly performing nightly exercises based on his own flawed theory.