Charles Darwin's collecting and accounting of compelling evidence for the effect of natural selection on the evolution of species necessarily emphasized the structure of animal species. But species may be characterized as well by their behavior and, perhaps, by their thoughts and feelings. Darwin knew the potential power of this idea, for his last book was concerned with the evolution of human and animal emotions.
Some readers believed that the nature of animals' structures could be used to erect a metaphorical ladder, this being a hierarchy of animals organized, for example, by order, genus, and species. If such a ladder of physical structure could be organized, could we also organize a mental ladder, one that arranged animal life by its kind or quality of thinking or feeling?
The idea of a Mental Ladder is similar to that of the Chain of Being, but the apparent similarities may deceive us. To illustrate, this part considers the importance attached to monkeys, our fellow primates, in the building of the Mental Ladder.