You will now read a short passage and listen to a lecture on the same topic. You

Narrator
Now listen to part of a lecture in a psychology class.
Professor
Let’s start with a physical attribute, say, uh, in kittens. Adult cats have extremely good vision, especially at night. But in order for a kitten’s eyesight to develop normally, the kitten must be exposed to light during the first four months of its life. Without that, its eyesight will not develop correctly, it will never be able to see as well as it should. Even if the kitten is exposed to plenty of light after those four months of darkness, it won’t matter, its vision will never develop normally.
As far as behavior’s concerned, well, have you ever seen how little baby geese line up and then, single-file, they follow their parent goose around? Well, what would happen if they didn’t see a parent goose within the first two days of their lives?
Actually, for normal behavior to develop, they must see what to follow within these first two days. What happens is, whatever large moving object they first see during those two days, they’ll adopt that object as their parent... forever. It can never be changed. For example, suppose after the baby geese were hatched, the only other animal around was, I don’t know, say a dog. OK? So the baby geese see a dog, but no other geese. Even though the dog is a totally different species, the geese will adopt it as their parent—they’ll follow it around. And even if the parent geese reappear later, it won’t matter to the babies—they’ll follow the dog. After two days the behavior is fixed and they’ll never exhibit the normal behavior of following their real parent—a goose.
Narrator
Using the examples of kittens and geese, explain the idea of a critical period.