There are four principles of modern biology. Even though there are many branches of biology, these four principles always hold true.
First Principle: Cells are the basic unit of life, make up all life, and come from other cells.
Second Principle: Genes are units of heredity, which means that they are passed down from father to son (or daughter).
Third Principle: Living things require a stable internal environment regardless of their surroundings.
Fourth Principle: Organisms require energy.
These are four principles of biology, and they are called that way because they all hold true, whether you're studying botany, zoology, or any other branch of biology.