An instance of type of biological taxon. This is the collection of all instances of species which are specializations of animal. The collection is defined by a rule that uniquely determines whether a type of animals is an instance of an instance of animal species or not at any given point in time. This rule is that: 1.) there are instances of the type that can breed. 2.) instances of the type that breed with each other can produce viable offspring which can also breed. 3.) such offspring are members of the same species as their parents. 4.) if a member of one animal species successfully breeds with an animal which is not a member of the species, no offspring that result would be fertile.

Thus, whether a type of organisms is an instance of animal species or not depends on what other collections of animals exist at the same time.

Historical cut-offs of the starting point of a species are conventional. The earliest member(s) of an animal species would be able to breed with some of their ancestors and their ancestors' kin.