Darlington, Cyril Dean (1903-1981) was a British biologist who formed and supported a number of basic ideas about chromosomes and their role in heredity. One of Darlington's main contributions was his help in clarifying the means by which a process of cell division called meiosis takes place. Darlington also held a deep interest in evolution, and he published a number of theories on how the mechanisms of reproduction and heredity are linked in evolution. At times, his theories sparked controversy among his colleagues in science, but they contributed greatly to our knowledge of chromosomes and meiosis.

As a young scientist, Darlington immersed himself in the study of the internal structure and organization of cells. At the time, little was understood about how cells divided and especially what happened to chromosomes during meiosis. Meiosis occurs with sex cells called gametes, which include sperm, pollen grains, and eggs. It results in the sex cells' having half the number of chromosomes that other cells in the organism have. In human beings, meiosis produces sperm and egg cells that have 23 chromosomes each. All other cells in the human body have 23 pairs of chromosomes, or 46 chromosomes. The other process of cell division, called mitosis, results in cells with 23 pairs of chromosomes in the human body. During Darlington's time, scientists understood that chromosomes paired up during cell division, but there was some controversy over whether chromosomes paired end-to-end or side-to-side during division and what was taking place during that pairing.

Darlington's mentor, Frank Newton, believed that meiotic chromosomes paired side-to-side and only later appeared to be paired end-to-end just as they were separating from their pairs. When Newton died in 1927, Darlington helped ready Newton's work for publication and continued his own research along the same lines. Like Newton, he used plants in his studies, including a specific type of hyacinth and a variety of evening primrose called Oenothera. In his research, Darlington found that chromosomes paired by segments rather than whole chromosomes. Following the hypothesis of scientist John Belling, he also concluded that there was an exchange of genetic material between the segments. Side-to-side pairing was later confirmed by other scientists.

Darlington also compared the earliest stages of both mitosis and meiosis and found that during mitosis chromosomes split into chromatids, the halves of a chromosome, and then undergo nuclear division (the division of a cell's nucleus). This allows pairing between chromatids. In contrast, during meiosis, the chromosomes begin nuclear division before they split. This finding was a milestone in the study of the cell nucleus and its structures, and it helped scientists understand how sex cells retain half the number of chromosomes after replication and division.

The mechanism called crossing over was yet another area of interest for Darlington. Crossing over is now known to be the exchange of genes between two partner chromosomes. During a crossing-over event, groups of genes from one chromosome change places with groups of genes from the partner chromosome. Through this process, different sperm or eggs may carry different combinations of linked genes. Darlington viewed crossing over not only as a means for genetic exchange between chromosomes, but as the key mechanism through which heredity and evolution took place.

Darlington became well known after the publication of his influential 1932 book titled Recent Advances in Cytology. The field of cytology is concerned with the formation, structure, and function of cells. In 1939, he published another important book, The Evolution of Genetic Systems, a survey of genetics that discusses the relationship between heredity and natural selection. His other books include The Evolution of Man and Society (1969), which was criticized by some for what they viewed as racist ideas, and The Little the Universe of Man (1978). He also helped found the journal Heredity in 1947.

Darlington was born in Chorley, England. He studied agriculture and science at Wye College and received a B.S. degree in 1923. He then joined the staff at the John Innes Horticultural Institution as head of the cytology department (1937-1939) and then as director (1939-1953). During his 30 years there he helped make the institute a world-renowned center of cytology. He then taught botany at Oxford University and managed the Oxford Botanic Garden, the oldest botanic garden in the United Kingdom. At Oxford, he established a research tradition in the study of cytogenetics, which examines the relationship between cells and heredity and variation. He remained at Oxford until his retirement in 1971.