Species

Collier's New Encyclopedia (1921)

in biology, a somewhat ambiguous 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 reproduction which, under similar conditions of existence, exhibit similar forms." Linnæus held that all species were the B 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 capability of reproduction. But besides varieties 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 resemblances varies in the case of different observers. At a later date was added the physiological definition that all the individuals of every species were capable of producing fertile offspring by intercrossing, whereas sexual intercourse between different species produced only sterile offspring or was actually infertile; and, though subject to exceptions, this definition is generally true. The descent of any given series of individuals from a single pair, or from pairs exactly similar to each other, is in no case capable of proof. Darwin in his "Origin of Species" says: "I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms" (see DARWIN- LAN THEORY). That book popularized the idea of the mutability of species, the chief factor in which Darwin believed to be Natural Selection, though he afterward modified his views to some extent as to its importance. A later theory of the origin of species is that of Physiological Selection, propounded by W. G. 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 infe forms-mutual sterility being thus regarded 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 another case the individual of which a species is predicated. Thus the individual, George, belongs to the logical species man, while man is an individual of the logical species animal. SPECIFIC GRAVITY. See GRAVITY, SPECIFIC. Cyc-VoL IX