In the Charles Darwin as the first anti-psychiatric thinker timeline, "Limits and conversion" is a book published by Charles Darwin in 1845 after being edited by Sandra Tanner.
The book "Limits and conversion" comments the response Charles Darwin got on the previous book "Slavery and psychiatry" of 1844, in which he argued that slavery and psychiatry were both scientifically unfounded and oppressive and that the definition of race and the definition of mental illness were both arbitrary.
Slavery advocates and psychiatrists both argued that while the exact limit was arbitrary, it was necessary to draw the line somewhere, or else society would descend into chaos. In "Limits and conversion", the case is made that relevant characteristics are measurable in their own right and does not have to hide behind arbitrary limits. Also, the theory of evolution allows for conversion, that organs evolved originally for one function can suddenly become useful for another function at a critical threshold. Therefore, it is argued, the theory of evolution did not have to imply any necessity of drawing arbitrary limits somewhere.