Generalizations in biology are almost invariably of a probabilistic nature. As one wit has formulated it, there is only one universal law in biology: "All biological laws have exceptions". This probabilistic conceptualization contrasts strikingly with the view during the early period of the scientific revolution that causation in nature is regulated by laws that can be stated in mathematical terms. Actually, this idea occurred apparently first to Pythagoras. It has remained a dominant idea, particularly in the physical sciences, up to the present day. (...) With Plato, it gave rise to the essentialism. For him, the variable world of phenomena was nothing but the reflection of a limited number of fixed and unchanging forms, the essences. These essences are what is real and important in this world. Constancy and discontinuity are the points of special emphasis for the essentialists. Variation is attributed to the imperfect manifestation of the underlying essences.
Darwin, one of the first thinkers to reject the essentialism (at least in part), was not at all understood by the contemporary philosophers (all of whom were essentialists), and his concept of evolution through natural selection was therefore found unacceptable. Genuine change, according to essentialism, is possible only through the saltatation origin of new essences. Because evolution as explained by Darwin, is by necessity gradual, it is quite incompatible with essentialism.
Western thinking for more than two thousand years after Plato was dominated by essentialism. It was not until the nineteenth century that a new and different way of thinking about nature began to spread, so-called population thinking. What is population thinking and how does it differ from essentialism? Population thinkers stress the uniqueness of everything in the organic world. What is important for them is the individual, not the type. They emphasize that every individual in sexually reproducing species is uniquely different from all others, with much individuality even existing in uniparentally reproducing ones. There is no "typical" individual, and mean values are abstractions. Much of what in the past has been designated in biology as "classes" are populations consisting of unique individuals.
From Ernst Mayr - The Growth of Biological Thought