Evolution (History and Scientific Foundation)

The world of organisms comprises a great system of individual forms generally classified according to structural resemblances into kingdoms, classes, orders, families, genera, species. The species is considered as the unit of the system. It is designated by a double name, the first of which indicates the genus, e.g. canis familiaris, the dog, and canis lupus, the wolf. Comparing the species of the present day with their fossil representatives in the geological layers, we find that they differ from one another the more the farther we retrace the geological record. To explain this remarkable fact two theories have been proposed, the one maintaining the stability and special creation of species, the other the instability and evolution, or genetic relation, of species. As is plain from the preceding section of this article, the principal difference between the two theories consists in this: that the theory of evolution derives the species of today by a progressive development from one or more primitive types, whilst the theory of constancy insists upon the special creation of each true species. It is generally admitted that the determination of genetic forms depends largely on the subjective views and experience of the naturalist.

We shall here continue our attention to the history and scientific foundations of the biological theory of evolution, leaving all purely philosophical and theological discussions to others. The entire subject will here be divided into the following parts: I. HISTORY OF THE SCIENTIFIC THEORIES OF EVOLUTION; II. DEFINITION OF SPECIES; III. VARIABILITY AND EXPERIMENTAL FACTS RELATING TO THE EVOLUTION OF SPECIES; IV. THE PALÆONTOLOGICAL ARGUMENT; V. THE MORPHOLOGICAL ARGUMENT; VI. THE ONTOGENETIC ARGUMENT; VII. THE BIOGEOGRAPHICAL ARGUMENT.

Before we begin, we wish to remind the reader of the important distinction brought out in the preceding essay, that the general theory referring to the mere fact of evolution must be well distinguished from all special theories which attempt to explain the assumed fact by ascribing it to certain causes, such as natural selection, the influence of environment, and the like. In other words, an evolutionist—that is, a defender of the general scientific theory of evolution—is not eo ipso a