This article explores the enabling intimacy between sentimentalism and biopolitics by turning to a less-than-obvious and yet characteristic example of the sentimental mode: the ubiquitous orphan tale of the mid- to late nineteenth century. It argues that individual orphan heroines of domestic plots not only function as tropes of domestic and national belonging, as has been widely recognized, but also of population regulation at the biological level of species. Sentimentalism functions as a mode of evolutionary theory, one that articulated the Lamarckian belief that sensory impressions shape the development of the individual body and of the species. Sentimental Lamarckism extended across literature, reform, and scientific theory and preceded naturalism’s deep engagement with evolutionary thought by decades. The sentimental orphan trope figures as a key aesthetic technology for regulating the growth of the population. Sentimental novels about orphans are not just about children who transform through their experiences; they were also directed at children readers and crafted to elicit emotional identification that could spur similar changes off the page.