For migratory birds, a change in the length of daylight hours is a signal to fly.
Most animals have eyes that are very different in size, structure, the ability to distinguish objects at different distances, to see during the day or at night. With all their differences, all organs of vision perceive light, therefore, they help animals to see the enemy or prey, drink or shelter, protect offspring or find a flower with nectar.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.