One explanation has been that many of an animal's traits are not fixed, but can change during its lifetime. This "phenotypic plasticity" enables individual animals to alter their appearance or behavior enough to survive in a new environment. Eventually, new adaptations promoting survival arise in the population through genetic changes and natural selection, which acts on the population over generations. This is known as the "Baldwin effect" after the psychologist James Mark Baldwin, who presented the idea in a landmark paper published in 1896.