Constancy of Species

See also Species.

Aristotle: species
Linnaeus: fixity of species

Compare Baraminology (evolution within the Biblical kinds) and Was the Flood followed by rapid evolution?

Mainstream scientific perspective

USA National Center for Science Education report at Species, Kinds, and Evolution. A quote (with our internal links):

Creationists will often claim that they are not interested in the species level, though. Initially, creationism did require fixity of species. In the 1920s, when George McCready Price equated "species" to the biblical "kinds", he was forced, to allow for the Ark to carry "every kind", to raise the bar higher. Even this was not original. In the late 18th century, Buffon, Cuvier's predecessor, had suggested that there was a "first stock" from which all members of a kind had evolved, so that all cats evolved from an original animal, modified by geography and climate, for instance. So creationists themselves have a "vagueness problem" no less than evolutionary biology does. Life is vague. Certainly the creationist "kind", or "baramin", as they mangle the Hebrew for "created kind", is extremely elastic. Given that elasticity, the motivation for the inference that was made naturally during the 17th and 18th centuries that species do not evolve is undercut. If kinds are not exact in reproduction, why think that the Genesis account is enough to prohibit evolution? The answer is, of course, that biblical literalism is not the primary motivation here for opposition to evolution.

Creationist perspective

An essay from the "creationist" point of view published by "Answers in Genesis" (well-known organisation that opposes evolution) is "Fixity of Species, A lesson in changing definitions". A short extract:

Others such as Basil, prior to the Latin Vulgate, discussed species as the biblical kind in the 4th century in his Homilies on Genesis 1. Matthew Henry, in the late 1600s and early 1700s, uses species as kinds in his notes on Genesis 2:3, saying there would be no new “species” created after creation week had completed. The list could continue. The point is that species originally meant the biblical kind. . . . 

After Linnaeus, both of these words (species and genus) were commonly used in modern biological classification systems with slightly different definitions. In the mid-to-late 1700s, species began taking on a new, more specific definition in scientific circles as a biological term (that definition is still being debated even today). But by and large, the definition had changed so that, instead of there being a dog species (or dog kind), there were many dog species.