Now the first attempts resulted in artificial classifications, much like our grouping of bats with birds and whales with fish. All animals, like coral animals and starfishes, whose similar parts were arranged in lines radiating from a centre, were united as radiates, however much they might differ in internal structure and grade of organization. But this radiate structure proved again to be largely a matter of adaptation.

Practically all animals having a heavy calcareous shell were grouped with the snails and oysters as mollusks. But the barnacle did not fit well with other mollusks. Its shell was entirely different. It had several pairs of legs; and no mollusk has legs. The barnacle is evidently a sessile crab or better crustacean. Its molluscan characteristics were only skin-deep, evidently an adaptation to a mode of life like that of mollusks. The old artificial systems were based too much on merely external characteristics, the results of adaptation. When the internal anatomy had been thoroughly studied their groups had to be rearranged.

Reptiles and amphibia were at first united in one class because of their resemblance in external form. Our common salamanders look so much like lizards that they generally pass by this name. But the young salamander, like all amphibia, breathes by gills, its skeleton differs greatly from, and is far weaker than, that of the lizard, and there are important differences in the circulatory and other systems. Moreover, practically all amphibia differ from all reptiles in these respects. Evidently the fact that the alligator and many snakes and turtles (of which neither the young nor the embryos ever breathe by gills) live almost entirely in the water, is no better reason for classifying

these with amphibia than to call a whale a fish, and not a mammal, because of its form and aquatic life.

When the comparative anatomy of fish, amphibia, and reptiles had been carefully studied it was evident that the amphibia stood far nearer the fish in general structure, while the higher reptiles closely approached birds. Then it was noticed that our common fish formed a fairly well-defined group, but that the ganoids, including the sturgeons, gar-pikes, and some others, had at least traces of amphibian characteristics. Such generalized forms, with the characteristics of the class. less sharply marked, were usually by common consent placed at the bottom of the class. And this suited well their general structure, while in particular characteristics they were often more highly organized than higher groups of the same class.

The paleontologist found that the oldest fossil forms. belonged to these generalized groups, and that more highly specialized forms-that is, those in which the special class distinctions were more sharply and universally marked-were of later geological origin. Thus the oldest fish were most like our present ganoids and sharks, though differing much from both. Our common teleost fish, like perch and cod, appeared much later. The oldest bird, the archeopteryx, had a long tail like that of a lizard, and teeth; and thus stood in many respects almost midway between birds and reptiles. And most of the earliest forms were 66 comprehensive," uniting the characteristics of two or more later groups. Thus as the classification became more. natural, based on a careful comparison of the whole anatomy of the animals, its order was found to coincide in general with that of geological succession.

Then the zoologist began to ask and investigate how the animal grew in the egg and attained its definite form. And this study of embryology brought to light many new and interesting facts. Agassiz especially emphasized and maintained the universality of the fact that there was a remarkable parallelism between embryos of later forms and adults of old or fossil groups. The embryos of higher forms, he said, pass through and beyond certain stages of structure, which are permanent in lower and older members of the same group.

You remember that the fin on the tail of a fish is as a rule bilobed. Now the backbone of a perch or cod ends at a point in the end of the tail opposite the angle between the two lobes, without extending out into either of them. In the shark it extends almost to the end of the upper lobe. Now we have seen that sharks and ganoids are older than cod. In the embryo of the cod or perch the backbone has, at an early stage, the same position as in the shark or ganoid; only at a later stage does it attain its definite position.

So Agassiz says the young lepidosteus (a ganoid fish), long after it is hatched, exhibits in the form of its tail characters thus far known only among the fossil fishes of the Devonian period. The embryology of turtles throws light upon the fossil chelonians. It is already known that the embryonic changes of frogs and toads coincide with what is known of their succession in past ages. The characteristics of extinct genera of mammals exhibit everywhere indications that their living representatives in early life resemble them more than they do their own parents. A minute comparison of a young elephant with any mastodon will show this most fully, not only in the peculiarities of their teeth,

but even in the proportion of their limbs, their toes, etc. It may therefore be considered as a general fact that the phases of development of all living animals correspond to the order of succession of their extinct representatives in past geological times. The above statements are quoted almost word for word from Professor Agassiz's "Essay on Classification." The larvæ of barnacles and other more degraded parasitic crustacea are almost exactly like those of crustacea in general. The embryos of birds have a long tail containing almost or quite as many vertebræ as that of archaeopteryx. But most of these never reach their full development but are absorbed into the pelvis, or into the " ploughshare" bone supporting the tail feathers. Thus older forms may be said to have retained throughout life a condition only embryonic in their higher relatives. And the natural classification gave the order not only of geological succession but also of stages of embryonic development. Thus the system of classification improved continually, although more and more intermediate forms, like archeopteryx, were discovered, and certain aberrant groups could find no permanent resting-place.

But why should the generalized comprehensive forms stand at the bottom rather than the top of the systematic arrangement of their classes? Why should the system of classification coincide with the order of geologic occurrence, and this with the series of embryonic stages? Above all, why should the embryos of bird and perch form their tails by such a roundabout method? Why should the embryo of the bird have the tail of a lizard? No one could give any satisfactory explanation, although the facts were undoubted.

Mr. Darwin's theory was the one impulse needed to crystallize these disconnected facts into one comprehensible whole. The connecting link was everywhere common descent, difference was due to the continual variation and divergence of their ancestors. The classification, which all were seeking, was really the ancestral tree of the animal kingdom. Forms more generalized should be placed lower down on the ancestral tree, and must have had an earlier geological occurrence because they represented more nearly the ancestors of the higher. But this explains also the facts of embryonic development.

According to Mr. Darwin's theory all the species of higher animals have developed from unicellular ancestors. It had long been known that all higher forms start in life as single cells, egg and spermatozoon. And these, fused in the process of fertilization, form still a single cell. And when this single cell proceeds through successive embryonic stages to develop into an adult individual it naturally, through force of hereditary habit, so to speak, treads the same path which its ancestors followed from the unicellular condition to their present point of development. Thus higher forms should be expected to show traces of their early ancestry in their embryonic life. Older and lower adult forms should represent persistent embryonic stages of higher. It could not well be otherwise.

But the path which the embryo has to follow from the egg to the adult form is continually lengthening as life advances ever higher. From egg to sponge is, comparatively speaking, but a step; it is a long march from the egg to the earthworm; and the vertebrate embryo makes a vast journey. But embryonic life is