work was that species gradually arose from other species through the accumulation of small variations. Varieties he designated as incipient species, and he thought that just as varieties might in time come to be distinct species, so species might become sufficiently different to be members of different genera. The theory of evolution is simply a consequence of the extension of our ordinary observations on the derivation of organic beings. The origin of organisms by generation from other organisms is the only kind of derivation that we know, and if there ever has been any other kind, the burden of proof naturally falls upon those who maintain that thesis. No one, so far as I know, has ever claimed to have seen a species created. Neither has any one seen a species evolved from other species, but within a relatively few generations man has witnessed the origin of numerous mutations, and he has produced extensive modifications in many kinds of plants and animals. There is no reason to doubt that species, genera, families, and larger subdivisions might have slowly arisen through the accumulation of differences of the kind which have actually been observed to arise. The geologist who observes stratified rocks which total many miles in thickness, and who is familiar with the way in which sedimentary deposits slowly accumulate at the bottom of the sea does not hesitate to regard these rocks as the result of the gradual accumulation of sediment, even though the process must have required millions of years. If an uneducated peasant is asked concerning the origin of the hills, mountains, and valleys of his environment he would probably reply that they have always been much as they now are ever since the beginning. The geologist, on the other hand, sees the story of the earth's evolution written in the rocks. He interprets the great changes of the past in terms of the small changes taking place in the present. His scientific imagination enables him to discern the historic meaning of the present configuration of the earth. He requires no sudden cataclysms or miraculous interventions in order to explain the successive transformations of the earth's crust; he can account for them, in an entirely satisfactory way, as the result of known causes of geological change.

In a somewhat similar manner the biologist pictures organic evolution as brought about by the gradual development of one kind of organism from another through the accumulation of such changes as may be seen to occur at the present time. He thinks of living beings as having slowly evolved through long geological ages from lower to higher types and finally culminating in the development of man. How do the phenomena of organic life fit into the biologist's picture?

C. THE EVIDENCE FROM CLASSIFICATION

Let us consider first the facts of classification, or the way in which organisms fall into groups. Owing to the vast number of different kinds of plants and animals found in various parts of the earth, the task of describing and classifying species has long formed one of the chief occupations of botanists and zoölogists. The great Swedish naturalist Linnæus initiated a number of reforms in preceding schemes of classification, one of these reforms being the system of binomial nomenclature, in accordance with which each species is given two names, one designating the genus, the other the species. Thus in Ursus americanus, the name of the common black bear of North America, Ursus represents the genus, and americanus the species. A genus includes a number of related species. In the genus Ursus, for instance, there is also Ursus horribilis, the grizzly bear, Ursus arctos, the European brown bear, and many other species in different parts of the world. Linnæus recognized that a species might give rise to varieties, but he held that these cannot diverge indefinitely, as they are confined by their nature within certain prescribed limits of variability. "We reckon," he says, "as many species as issued in pairs from the hands of the Creator." In agreement with the English naturalist, John Ray, Linnæus came to hold that species differ from varieties in being infertile when crossed, or in producing progeny which, like the mule, are sterile, so that each species is enabled to retain its distinctive characteristics with no more than a very limited amount of variation. This conception of species became the dominant

one among systematic botanists and zoölogists up to the time of Darwin.

Linnæus essayed the task of describing and classifying all the known species of plants and animals in his great work entitled the Systema Naturæ. The Systema Naturæ ran through twelve editions in the lifetime of its author, and, although the work naturally grew bigger with each edition, it was still possible in the latest issue to include brief descriptions of all known species of plants and animals within the limits of a single work. Linnæus clearly perceived that the classification of species should not be carried on in an arbitrary way, although he employed an artificial system in the grouping of plants. There is, he contends, a "natural," or true sytem of classification which it is the function of naturalists to discover and embody in their systems. We may arrange books in a library according to several artificial methods, such as the nature of the contents, the names of the authors, the color of the bindings, or in any other way suitable for the purpose in hand, but living beings fall into natural groups of their own accord. The various kinds of cats previously mentioned really belong together, regardless of whether they are large or small, or gray, black, or yellow. The species closely allied to our domestic cats are placed in the genus Felis, a group which contains the lion, the tiger, the puma, the leopard, and a considerable number of other species in several countries of the globe. The genus Felis, together with the allied genus Lynx and some others, constitute the Felidæ, or cat family, which is characterized by possessing many common features of structure and behavior. Any one would easily recognize these animals as really belonging together in any rational system of classification. According to the same general method of grouping, the Felidæ, the Canidæ (which includes the dogs, wolves, jackals, and their relatives), the Ursidae (or bear family), the Viverridæ (or civet family), etc., are placed together in the order Carnivora, or flesh-eaters, a group characterized by a general similarity of dentition and many other common features of organization.

Various other orders, such as the Rodentia (rats, mice, rabbits, etc)., the Ungulata (horses, deer, cattle, etc.), the Chiroptera, or bats, constitute the class of Mammalia, the members of which possess many structural peculiarities in common, chief among which are the mammary glands for supplying the young with milk. Corresponding to the class of mammals there are other groups of animals the birds, reptiles, amphibians, and fishes— which agree in possessing a vertebral column, dorsal nerve cord, red blood, and several other fundamental features of structural organization by virtue of which they are all included within the large group known as the Vertebrata. And this extensive group, or subphylum, along with the tunicates, Amphioxus, and a few other primitive forms that possess a dorsal nerve cord and a notochord in some period of their life, constitute the phylum Chordata.

The Chordata represent one of the basic subdivisions of the animal kingdom, but coördinate with this great group there are the Mollusca, Echinodermata (starfish, sea urchins, etc.), Cœlenterata (hydroids, jellyfish, coral polyps), Arthropoda (insects, crustaceans, spiders, etc.), and several other phyla each of which exhibits a distinct fundamental plan of structure. If we pass to the plant world we find a similar grouping of forms into corresponding categories of classification.

The arrangement of organisms into groups within group3 leads to a system of classification which has often been compared to the branching of a tree. The larger branches correspond to the phyla; the smaller ones to the classes, orders, and families: and the terminal twigs to the species. Some of the older naturalists endeavored to arrange organisms in a linear series, but it is quite evident that species cannot be made to fit into such a system. That they fall naturally into the treelike system of grouping is very readily explained if we assume that they are the product of gradual divergence along with descent from a common ancestor. The general features that characterize the members of any group would then be explained as due to a common inheritance from some ancestral species. As Darwin remarks,

Expressions such as that famous one by Linnæus which we often meet with in a more or less concealed form, namely, that the characters do not make the genus, but that the genus gives the characters, seem to imply that some deeper bond is included in our classifications than mere resemblance. I believe that this is the case, and that community of descent-the one known cause of close similarity in organic beingsis the bond, which, though obscured by various degrees of modification, is partially revealed to us by our classifications.

When we observe people living in the same region who look very much alike we commonly infer that they are related, i. e., we appeal to community of descent as an explanation of their resemblance. Likewise we assume that the resemblances within the racial stock to which they belong are due to the same cause. No one supposes that the Scandinavians, French, Italians, and Greeks, are the descendants of separately created ancestors. All of these people belong to the Caucasian race, and their similarities are always attributed to a common, though remote, ancestry. The human race in general falls into the same system of groups within groups that is exhibited by the subdivisions of plants and animals. Likewise the languages spoken by different peoples and races fall into the same type of arrangement. From the different dialects of the same country there are all degrees of difference in speech up to such distinct languages as Latin and Chinese. In many cases, as may be illustrated by the Romance languages (French, Italian, Spanish, and Portuguese), it is a matter of history that the divergent tongues arose by a process of evolution, and linguists are now in general agreement that all languages developed in the same way.

Naturalists have often commented on the fact that different characters have very different values for purposes of classification. If we classify organisms according to an arbitrarily chosen character, we may group together forms which differ profoundly in all other respects except in possessing the character selected. Were we to put into one group all of the legless vertebrates, we should include such creatures as lampreys, eels (which are limbless fish), snakes, and certain legless lizards and amphib