At the dawn of the twentieth century, the new field of comparative psychology was being heavily influenced by British naturalist Charles Darwin's theory of evolution. In 1872, Darwin had published a book titled The Expression of the Emotions in Man and Animals that was a forerunner of later writings on comparative psychology. In this book, Darwin suggested that many expressions of emotion are genetically based rather than learned. As such, they are the product of evolution, and their roots can be traced back to humankind's shared past with other animals. Therefore, neither emotions nor their expression are uniquely human. Darwin believed that other animals may experience some of the same emotions as humans. These animals also may display those emotions in ways that resemble the facial expressions and gestures of people.
Today, scientists are divided on whether Darwin was right about this matter. Some prefer to consider the expressions of animals strictly as communication signals. Others have no qualms about attributing emotions to animals. Even among the latter group of scientists, however, there is no clear agreement about which emotions animals feel, and whether the animals experience those feelings in the same way as humans do.
Whatever the final verdict on emotions in animals, Darwin's ideas raised a crucial question: Should humans be seen as just one species among many? Or should they be seen as unique and distinct from other animals? Many comparative psychologists, who wanted to draw parallels from animal to human behavior, took the first position. Wundt, for instance, wrote that "the mental life of animals shows itself to be throughout, in all its elements and in the general laws governing the combination of the elements, the same as the mental life of man." Others, however, cautioned against anthropomorphism—ascribing human thoughts and feelings to nonhuman animals.
Darwin also wrote about the process of natural selection, in which the fittest members of a species tended to survive longer and produce more offspring than other members. In the post-Darwin era, scientists began to look for genetically programmed behavior patterns that might aid survival. Such patterns, which develop without the need for learning, are known as instinctive behaviors. For example, Morgan studied newborn chicks and ducklings hatched in an incubator. He found that pecking, walking, scratching, preening, stretching up and clapping the wings, scattering and crouching when alarmed, and making a danger sound were all inborn behavior patterns. Chicks and ducklings arrived in the world already programmed to display such behaviors, although they sometimes needed practice to reach a high level of skill.
The evolution of behavior remains a core concept in comparative psychology to this day. While most psychologists now accept that some behaviors are instinctive, they also stress that the way these behaviors are expressed can be affected by development and experiences.
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