First, there is what is called the Reflex Theory.

This holds that instincts are reflex actions, like the closing of the eye when an object threatens to enter it, only much more complex.

They are due to the compounding and adding together of simple reflexes, in greater and greater number, and with increasing efficiency.

This theory attempts to account for instinct entirely in terms of nervous action.

It goes with that view of evolution which holds that the nervous system has had its growth from generation to generation by the continued reflex adjustments of the organism to its environment, whereby more and more delicate adaptations to the external world were secured.