SPECIES, in biology, a somewhat am biguous term used to denote a limited group of organisms, resembling each other, and capable of reproducing similar organisms, animal or vegetable, as the case may be. A species is defined by Haeckel as "the sum of all cycles of re production which, under similar condi tions of existence, exhibit similar forms." Linnmus held that all species were the direct descendants from and had the characters of primevally created forms, and in this he was followed by those who accepted the first chapter of Genesis in a strictly literal sense. Buffon and Cuvier, leaving the question of origin on one side, held the distinguishing marks of a species to be similarity and capabil ity of reproduction. But besides varie ties and races in various species of animals and plants, dimorphism, and in others trimorphism, exists, so that close similarity cannot be taken as a criterion, and the value attached to external re semblances varies in the case of differ ent observers. At a later date was added the physiological definition that all the individuals of every species were capable of producing fertile offspring by inter crossing, whereas sexual intercourse be tween different species produced only sterile offspring or was actually infer tile; and, though subject to exceptions, this definition is generally true. The de scent of any given series of individuals from a single pair, or from pairs exactly similar to each other, is in no case capa ble of proof.
Darwin in his "Origin of Species" says: "I look at the term species as one arbitrarily given for the sake of con venience to a set of individuals closely resembling each other, and that it does not essentially differ from the term va riety, which is given to less distinct and more fluctuating forms" (see DARwIN IAN THEORY). That book popularized the idea of the mutability of species, the chief factor in which Darwin believed to be Natural Selection, though he after ward modified his views to some extent as to its importance. A later theory of the origin of species is that of Physi ological Selection, propounded by W. G. J. Romanes, F. R. S., who holds that many species have arisen on account of variations in the reproductive system, leading to some infertility with parent forms—mutual sterility being thus re garded as one of the conditions, and not as one of the consequences of specific differentiation.
In logic, a predicable that expresses the whole essence of its subject in so far as any common term can express it. The names species and genus are merely relative, and the same common term may in one case be the species which is predicated of an individual, and in an other case the individual of which a species is predicated. Thus the individ ual, George, belongs to the logical spe cies man, while man is an individual of the logical species animal.