Systemic circulation of blood and double circulatory system by William Harvey Observation of microorganisms by Antony van Leeuwenhoek Discovery of the role of oxygen in respiration and photosynthesis by Joseph Priestley, Antoine Lavoisier and Jan Ingenhousz Former existence of extinct species Discovery of evidence for the evolution of species in the fossil record and DNA sequences Chromosomes in the cell nucleus bear a definite linear arrangement of genes - the Chromosome Theory of Inheritance. Discovery of the essential minerals, nutrients and vitamins in foodstuffs Discovery of the link between arteries and veins by Marcello Malpighi Discovery of the animal reproduction by Lazzaro Spallanzani Discovery of the animal electricity by Luigi Galvani The role of microorganisms in causing infectious disease, by Louis Pasteur and Joseph Lister The nervous system acts via electrical impulses Discovery of the structure and function of DNA and RNA Discovery of the structure and function of enzymes and other proteins Discovery of Nerve growth factor by Rita Levi-Montalcini and Stanley Cohen Discovery of restriction endonucleases, later used in genetic engineering, by Daniel Nathans and Hamilton Smith Discovery of the Impermanence of Sexual Phenotypes as to the conclusive statement, by Isidro A. T. Savillo