The Existence of Species

Chapter IV — The Origin of Species

Article 1. The Existence of Species

This Chapter discusses the wondrously harmonious yet varied groups of living bodies found in the world, and studies the gradation observable among them. It seeks to account for the order and apparent relationship existing among these organisms. It tries to explain the presence of organic types which were not found on the earth in an earlier period, and to discover the connection between these types and others which were once in existence but which have long since disappeared. The Chapter presents and studies various theories which have been advanced to explain the present state of organic life on the earth. These matters are discussed in two Articles:

Article 1. The Existence of Species
Article 2. The Problem 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. And in current daily speech the word species is almost synonymous with sort or kind. In the present study we employ the term species in the biological sense.

Species, in the biological sense, means a definitely marked group of living bodies. It means a class or group of living things (plants or animals—but we refer it here chiefly to animals) which is distinguished from other such groups by the structure of its members (their bodily “build”) and their capacity for interbreeding. Many definitions of species have been formulated by biologists, and no one of them is entirely acceptable to all scientists. Professor Bateson offers us this sonorous definition: “A species is a class (of organisms) marked by morphological discontinuity and interspecific sterility.” Morphological discontinuity indicates the fact that the body-structure of members of one species is unlike (or discontinuous with) that of members of another species: in a word, there is dissimilarity of “build” and bodily form between members of differing species. Interspecific sterility means that a female member of one species and a male member of another species cannot produce offspring. Professor Poulton formulates a very simple definition of species; he calls it, “An interbreeding community.”

Whatever be the best definition of species, all agree that the most notable marks of such a class are two: similarity of structure, and,—apart from physical incapacity due to difference in size,—the capacity for having offspring indefinitely in the 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.

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”).

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.”

A species differs from every other species, as we have seen, in point of structure and filiation (from Latin filiatio “the having of offspring”). But many species may be grouped together on the basis of common characteristics more general than the specific determinants. Such a group of species is called a genus. And a genus grouped with other genera forms a family. And families of a type constitute an order. Orders are grouped into classes, and classes into phyla. The phylum is the most general class of organisms recognized by the biologist.

b) Variety and Multiplicity of Species

There is no need of argument or explanation to establish the fact that this earth of ours is inhabited by a bewildering number and variety of living bodies. Bird, and beast, and reptile, and fish, and insect, are types of organisms which we all know and recognize; and the earth is a veritable wonderland of varied plants. Even the least observant among us notices a great variety of types among animals and plants, and recognizes differences among the members of the same general type. The student who has had to study botany or biology, and has been put to the task of classifying the members of one order (such as the beetles), or of one genus, will be much more deeply impressed than the ordinary man by the variety and multiplicity of organisms, and by their amazing unity, harmony, arrangement, and gradation.

Geologists,—those scientists who study the strata of the earth-crust on which we live,—show us proof that the earth was not always the home of living bodies. The fossil remains of plants and animals furnish further proof that, when life appeared, 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.