ians a heterogeneous assemblage of animals really belonging to several different classes. If, on the other hand, we based our grouping on the possession of hair we should include nothing but the members of the class of mammals. The value of a character for classification depends upon the extent to which it is an index of the presence of other features of organization. If an animal has true hair it will also have mammary glands, a four-chambered heart and warm blood. If it is simply devoid of legs, we cannot safely predicate what other characters it may have, as it may be a snake, an eel, or even an angle

worm.

It has long been recognized also that the value of a character for classification is not determined by its functional importance; in fact, as De Candolle has remarked, it often stands in inverse relation to its functional importance. Rudimentary organs are considered especially valuable as indications of affinity. The presence of rudimentary limbs in certain snakes is a good evidence of affinity with animals bearing limbs, since the only rational interpretation of these rudiments is afforded by the theory that snakes descended from ancestors in which these appendages were functional.

The grouping of organisms in the natural system of classification is one which would be inevitably brought about by descent with modification, as is exemplified in the origin of races, varieties, languages, and other admitted products of evolutionary changes. Only an arbitrary line can be drawn between varieties and species. Varieties are related to species as species are to genera, genera to families, families to orders, and so on. Linnæus, later in life, came to hold that the species of a genus descended from a common created form, but if we assume a common descent for the species of a genus, we can with as much reason make the same assumption for the genera of a family, or the families of an order. The position of Linnæus is much like that of a geologist who would admit that the natural processes of sedimentation might produce strata ten feet thick, but that they could not produce strata a hundred feet thick.

D. THE EVIDENCE FROM MORPHOLOGY

The argument for evolution based on morphology, or the science of structure, has much in common with the argument from classification. It is based upon the resemblance of organisms in structure and the ability of the theory of evolution to explain these resemblances as due simply to inheritance from a common ancestor. As we have already seen, the members of any natural group possess a common plan of organization which is retained, no matter how great the changes which have occurred in adaptation to diverse conditions of life. In the great class of insects, for instance, the most extensive group in the animal kingdom, this fundamental plan is always clearly manifest. The same number of segments is apparently retained throughout the group. Herbert Spencer, in commenting on this circumstance, pertinently asks:

Why under the down-covered body of a moth and under the hard wing-cases of a beetle, should there be discovered the same number of divisions? Why should there be no more somites in the Stick-insect, or the Phasmid a foot long, than there are in a small creature like the louse? Why should the inert Aphis and the swift flying Emperorbutterfly be constructed on the same fundamental plan? It cannot be by chance that there exist equal numbers of segments in all these multitudes of species. There is no reason to think it was necessary, in the sense that no other number would have made a possible organism. And to say that it is the result of design-to say that the Creator followed this pattern throughout, merely for the purpose of maintaining the pattern-is to ascribe an absurd motive.

The same principle of unity of plan is beautifully exemplified in the Crustacea. Consider such a creature as the crayfish or the lobster. The body, as in the other members of the order Decapoda to which these forms belong, is composed of twenty segments. Most of these segments bear each a pair of appendages whose plan ofcture is the same, although the appendages are modified the very di se functions of smell, touch, mastication,

ing eggs, and swimming. In studying

these organs one seems almost compelled to think of them as resulting from the transformation of a series of originally similar appendages. This interpretation is amply confirmed when we compare them with corresponding organs of the more primitive crustaceans. In the latter, certain organs which in the lobster are used for mastication are employed for ordinary locomotion. Among the crustaceans we find that where more organs are used for locomotion fewer are used for mastication, and vice versa. There seems to be no good physiological, or teleological reason why crustaceans with a large number of locomotor appendages require a less number for chewing their food, but this relation, like many others, is intelligible if we assume that these forms are descended from a common ancestral type. In all of the crustaceans, the jaws and accessory jaws (mandibles and max

illæ) are homologous with legs, and it is of interest to observe that in that living fossil, the horseshoe crab (which has very little in common, by the way, with the true crabs), there are no special organs of mastication, this function being performed by the basal joints of the ordinary walking appendages.

A striking instance of the diverse modifications of organs having a common plan of structure is afforded by the limbs of vertebrate animals. In the vertebrates above the fishes the limbs may readily be conceived as modifications of a common pentadactyl type. In the wing of a bat we find four of the digits enormously elongated to form supports for the web of the wing, the first digit remaining relatively unmodified. In the extinct

pterodactyls only the outer, or fifth, digit is elongated, the three others being of the ordinary prehensile type. In the wing of the bird only three digits are represented, although a fourth is discernible in

the embryo. The flying organs in these very distantly related groups are formed on the same fundamental plan as the fore limb of a cat or the arm of a man. They are all readily explicable as having arisen from the modification of a generalized type of pentadactyl limb, the parts of which have been employed in different ways to adapt the organ for the function of flight. We have in these cases an excellent illustra

TURTLE

OWL

N.

GALEUS

FROM INSIDE

EAGLE
FROM

INSIDE

usually provided for FIG. 185-Nictitating membrane (third eyelid) N, in the eyes of vertebrate animals. (After Romanes.)

by the transformation

of old structures. The sucking mouth parts of insects are not organs sui generis, but they represent transformations of mouth parts of the mandibulate type fitted primarily for chewing. And the chewing mouth parts of insects are homologous with locomotor appendages, as they are in the Crustacea.

There are probably no more striking evidences of descent than the rudimentary organs which abound in the structure of most

higher animals. We have previously remarked upon the many species with rudimentary eyes, which live in caves. In the inner corner of our own eye there is a small fold, the semilunar fold, which is homologous with the third eyelid, or nictitating membrane of birds and reptiles (Fig. 185). If you watch for a time the eye of a domestic fowl you may observe a semi-transparent membrane which is drawn at intervals across the eye from the inner angle. This third eyelid is functional in most of the birds and

reptiles, but it is rudimentary in nearly all the higher mammals

as it is in ourselves. We have already mentioned the curious third eye which persists as a rudiment in several species

FIG. 187-External ears of different species of primates. (After Romanes.)