Cassandra's mother told her, "you know you are in love when your heart beats fast and you experience that unique trembling feeling inside." this remark best illustrates the theory of emotion.

James Lange theory of emotions

Explanation:

James Lange's theory of emotions proposed that emotions are equivalent to the physiological arousal that leads to emotions like fear, anger, happiness, etc that are caused by external factors.  

This theory was opposed to the common-sense theory of experience in which cause and effect relation saw that manifested the experience of emotions. The autonomic activities and the physiological activities induced the feeling of emotions.  

For instance:

Angry bull-physiological reaction-feeling of fear

answer; with few regulations protected the prices of crops which made farmers leave their farms and migrate to the big cities to work in factories;