The Subtle Code of Inequality in Children's Books:
Pigs in particular are substantially over-represented in subordinate jobs (those with low skill and no authority), where their overweight bodies and (judging from the plots of these books) congenital stupidity seems to "naturally" equip them for subservient jobs. Here, see this additional image from Scarry's book, showing construction work being performed by the above-mentioned swine.
In effect, Martin's point is that there is a hidden language or code inscribed in children's books, which teaches kids to view inequalities within the division of labor as a "natural" fact of life – that is, as a reflection of the inherent characteristics of the workers themselves. Young readers learn (without realizing it, of course) that some species-beings are simply better equipped to hold manual or service jobs, while other creatures ought to be professionals. Once this code is acquired by pre-school children, he suggests, it becomes exceedingly difficult to unlearn. As adults, then, we are already predisposed to accept the hierarchical, caste-based system of labor that characterizes the American workplace.