During the late 1950s biologists began to study the nature of art in humans. Theories were proposed based on observations of non-human primate paintings. Hundreds of such paintings were cataloged by Desmond Morris. Morris and his associate Tyler Harris interpreted these canvas paintings as indications of an intrinsic motivation toward abstract creativity, as expressed through an exploration of the visual field and color. Many of these painters progressed over time by expanding or contracting the area of paint coverage, the horizontal or vertical stroke relationships, and even the development of content. Such paintings were exhibited in many modern art museums during an early 1960s and late 1950s. The cultural and scientific interest in these paintings diminished steadily and little note is taken today.