The Mental Capacity of Animals
In his aim to explain how the human mental capacities could be a result of evolution, Darwin uses the abilities of several animals in an allegorical way. He describes the ability of dogs to show compassion, the ability of apes to use tools or social dimensions of monkey society. With this comparison, Darwin tries to show that while human mental capacity is so much higher than those of any extant animal, the type of ability is the same. And if the basic concept is present, the theory of evolution easily explains its amplification.
The Difference between Human Races
One of the common motifs throughout the book is the problem of human races. While The Origin of Species, Darwin's first big hit, describes the forces of evolution, society was rather slow in the application of its truths to the human species. The intellectual elite was divided between those that said that humans all come from a common ancestor and those that argued that each race developed individually and should be considered a species. Darwin strongly argued for the former, using the gradual change between races geographically as his main evidence.
Due Care
One of the most striking symbols used in the book is the concept of due care. Darwin likens the propagation of a successful species to a surgeon who goes into the operation room prepared, or a farmer who takes care that only his healthiest animals have offspring. This argumentation, later used in the infamous ideology of eugenics, is a key point in The Descent of Man, and Selection in Relation to Sex. Darwin argues that human morality stops society from any cruel action against those that are weak of mind or body, but that time and a good education will allow the human race to reach its full potential.
A Hierarchy of Evolution
In stark difference to his original argument in The Origin of Species, Darwin proposes the idea of a hierarchy of evolution. He uses a symbolic scale to rank species based on their evolutionary success. This change is especially striking as he included the different human races on this scale, clearly valuing the European race above all others. Together with his influence on the theory of eugenics, this ranking is the basis for most pseudo-scientific modern racial discrimination.
The Feathers of a Peacock
While this symbol is only used as an example, it was the mental origin of the theory of sexual selection. A plumage so extravagant seemed to be in contradiction to the original theory of evolution, as it put more pressure on the individual animal than necessary. Darwin uses this symbol to explain his theory that throughout evolutionary processes, the choice of whom to mate with is an important factor. Animals develop certain traits to be more interesting to the opposite sex. Interestingly, after the publication this theory has been used to explain a lot of animal traits that have little to do with sexual selection but rather are adaptations to the animal's environment. However, such detailed knowledge about ecology was not yet available to the scientists.
