<P> Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being . In the 19th century, biologists grasped that species could evolve given sufficient time . Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection . That understanding was greatly extended in the 20th century through genetics and population ecology . Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures . Genes can sometimes be exchanged between species by horizontal gene transfer; new species can arise rapidly through hybridisation and polyploidy; and species may become extinct for a variety of reasons . Viruses are a special case, driven by a balance of mutation and selection, and can be treated as quasispecies . </P> <P> As a practical matter, species concepts may be used to define species that are then used to measure biodiversity, though whether this is a good measure is disputed, as other measures are possible . </P> <P> In his biology, Aristotle used the term γένος (génos) to mean a kind, such as a bird or fish, and εἶδος (eidos) to mean a specific form within a kind, such as (within the birds) the crane, eagle, crow, or sparrow . These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean terms thus named; today the birds are a class, the cranes are a family, and the crows a genus . A kind was distinguished by its attributes; for instance, a bird has feathers, a beak, wings, a hard - shelled egg, and warm blood . A form was distinguished by being shared by all its members, the young inheriting any variations they might have from their parents . Aristotle believed all kinds and forms to be distinct and unchanging . His approach remained influential until the Renaissance . </P> <P> When observers in the Early Modern period began to develop systems of organization for living things, they placed each kind of animal or plant into a context . Many of these early delineation schemes would now be considered whimsical: schemes included consanguinity based on colour (all plants with yellow flowers) or behaviour (snakes, scorpions and certain biting ants). John Ray, an English naturalist, was the first to attempt a biological definition of species in 1686, as follows: </P>

Who was the first to describe that species is the unit of classification