The time period being referred to is the sensitive period. This time period is also referred to as the critical period. The importance of a critical period is such that it must involve certain stimuli from which an organisms learns and acquires traits. If some of these stimuli are not present in the critical period, the organism has difficulty acquiring the trait or skill, and may even find it to be impossible to learn the skill.
Some of the evidence comes from fossils, and some comes from studies that show how similar living things are to one another. By the 1930s, scientists had also learned about genes. As a result, they could finally explain how characteristics of organisms could pass from one generation to the next and change over time
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