The Existence of Species
The meaning of biological species; the two marks that define it — structure and filiation; varieties, breeds, races, hybrids, and the biological classification of organisms.
A biological species is a natural class of living bodies distinguished from all other classes by two marks: common structure (morphological type) and filiation (the capacity for indefinitely fertile interbreeding within the group, and either infertility or reversion to type when crossing outside it). Within species, minor natural groups are varieties; varieties artificially cultivated become breeds or races. The offspring of parents of different races is a mongrel; that of parents of different species is a hybrid, which is typically sterile. The biological taxonomy of all living things runs from species upward through genus, family, order, class, phylum. The present enormous variety of species on earth is compounded by extinct species known from the fossil record — a diversity that poses the philosophical problem of origins discussed in Article 2.
a) Meaning of Species — b) Variety and Multiplicity of Species
a) Meaning of Species
The term species is used in a wide variety of meanings. It has one meaning for the student of Logic, another for the person who studies the knowing process (as we shall presently do in the second Part of this manual), and still another for the laboratory scientist or biologist. In the present Article we are using the term species in its biological meaning.
In biology, species means a class or group of organisms all of which are essentially alike in structure and nature, and all of which can breed indefinitely one with another in their natural or wild state. The members of a species have bodies built on the same general plan, and, when left in their natural (or wild) state, the species tends to perpetuate itself, and not to die out.
Species are distinguished from one another by two marks: structure and filiation.
Structure means the body-plan or morphological type. Members of the same species are built upon the same general plan; they are of the same morphological type.
Filiation (from the Latin filiatio “the having of offspring”) means the capacity of producing fertile offspring. Within the group called species there are minor groups called varieties. When varieties are artificially cultivated, they are called breeds or races. The offspring of parents of different races is called a mongrel. Now and then the descendants of a mongrel (perhaps after several generations) exhibit marked characteristics of one or other of the breeds in which the mongrel strain began. This sort of reversion or “throw back” is called atavism, a term derived from the Latin atavus “ancestor.”
Sometimes organisms of differing species may have offspring. Such offspring is called a hybrid. But the hybrid has no offspring, or, if it has,—and cases of the kind are extremely rare,—the offspring is not like the hybrid, but like one of the parents of the hybrid. Thus the offspring of a hybrid manifests what is called reversion or reversion to type (from the Latin reversio “a turning back”).
A species differs from every other species, as we have seen, in point of structure and filiation. But many species may be grouped together on the basis of common characteristics more general than the specific determinations. Such a group of species is called a genus (Latin “a kind”). Many genera may be grouped in a family; many families in an order; many orders in a class; many classes in a phylum (Greek “a tribe”). The largest possible groupings of living bodies are the phyla. These are sometimes called kingdoms or sub-kingdoms.
b) Variety and Multiplicity of Species
The variety of living organisms now existing on earth is truly astonishing. The number of known plant-species is very great. The number of known animal species is even greater. Some years ago the number of classified species of insects alone was stated at 360,000, and the figure was called incomplete. The number of mammals and birds is considerable. The number of fish-species runs to ten thousand or more.
To this enormous variety of species now in existence there must be added the species that have become extinct, the organisms that were once found on earth and have long since disappeared. There are many organisms found in the earliest geological strata (that is, in the earliest layers of rock that contain traces or imprints of living things) which are unlike anything now found in the world. The organisms were not of the types with which we are now familiar. There are many species of living bodies now in existence which did not exist in earlier times, and many species that were once here have disappeared. Out of this fact emerges the scientific and philosophical problem which we are to discuss in the next Article.
Summary of the Article
In this brief Article we have described species; we have offered several definitions of the term, and have noticed the two outstanding marks by which species are distinguished from one another, viz., structure and filiation. We have defined the terms hybrid, mongrel, reversion, atavism. We have listed the biological classification of living things from species back to phyla. We have indicated the great variety and multiplicity of species now existing and have called attention to the fact that the number of these is to be increased by the addition of extinct types of organisms.