This states that an organisms during its development tends to repeat the evolutionary history of its ancestors

Explanation

1) Fishes have the 2-chambered heart with an atrium and a ventricle. Blood passes through the heart only once in a complete circuit hence called single circulation. This means there is no separate circulation for oxygenated and deoxygenated blood.
2) Amphibians have a 3 - chambered heart with two atria and one ventricle, which further evolved in reptiles, have two atria and an incompletely divided ventricle in which left atrium receives oxygenated blood from the gills/ lungs/ skin and right atrium receives blood from the other parts of the body. The two types of blood get mixed in the single ventricle, which pumps out mixed type of blood. Thus these animals show complete double circulation.
3) Birds and mammals possess 4- chambered heart with two atria and two ventricles. In these animals the oxygenated and the deoxygenated types of blood received by left and right atria, passes on to the left and right ventricles, respectively. The ventricles pump the blood out without any mixing of the oxygenated and deoxygenated types of blood. Hence these snimals are said to be showing double circulation namely systemic and pulmonary circulations.