For 'adapted' we had better read 'adaptable,' because a variation which does not answer, which cannot be made use of, or, still more notably, is a hindrance or disadvantage, does not become an adapted feature. There is often a confusion between adaptation as an accomplished fact, a feature, or resultant condition, and adaptation as the mode of fitting the organism to, or making the best of, the prevailing surroundings or circumstances.
Étienne Geoffroy Saint-Hilaire was born in 1772 at Étampes, Seine-et-Oise. He was originally brought up for the Church; but when already ordained he attended lectures on natural science and medicine in Paris. He managed to get the place of assistant in the Musée d'Histoire Naturelle; he became Professor of Zoology in 1793, and took the opportunity of encouraging young Cuvier. Later he became Professor of Zoology of the Faculté des Sciences, and in 1818 he published his remarkable 'Philosophie anatomique.' He died in 1844.
He had conceived the 'unity of organic composition,' meaning that there is only one plan of construction,—the same principle, but varied in its accessory parts. In 1830, when Geoffroy proceeded to apply to the Invertebrata his views as to the uniformity of animal composition, he found a vigorous opponent in Cuvier. Geoffroy, like Goethe, held that there is in Nature a law of compensation, or balancing of growth, so that if one organ take on an excess of development, it is at the expense of another part; and he maintained that, since Nature takes no sudden leaps, even organs which are superfluous in any given species, if they have played an important part in other species of the same family, are retained as rudiments, which testify to the permanence of the general plan of creation. It was his conviction that, owing to the conditions of life, the same forms had not been perpetuated since the origin of all things, although it was not his belief that existing species were becoming modified. Cuvier, on the other hand, maintained the absolute invariability of species, which, he declared, had been created with regard to the circumstances in which they were placed, each organ contrived with a view to the function it had to fulfil,—thus putting the effect for the cause ('Encyclopædia Britannica,' 9th edition, vol. xxi., p. 171).
George [Cuvier] in the department of Doubs, which at that time belonged to Württemberg. He was educated at Stuttgart, and studied political economy. While acting as private tutor to a French family in France he followed his favourite pursuit, the study of natural sciences. Geoffroy Saint-Hilaire heard of him, and appointed him assistant in the department of comparative anatomy in the Musée d'Histoire Naturelle. In 1799 he was elected Professor of Natural History at the Collège de France, and soon after he became Perpetual Secretary of the Institut National. In 1831, a year before his death, Louis Philippe raised him to the rank of a peer of France.
Cuvier was the first to indicate the true principle upon which the natural classification of animals should be based—namely, their structure. It is the study of the anatomy of the creatures and their comparison which affords the only sound basis of a classification. The work which had the greatest influence upon the scientific public is his 'Règne animal distribué d'après son Organisation,' 1817. The system which he propounded in this book gradually came to have almost world-wide fame, and, in spite of its many obvious deficiencies, still lingers in some of our most recent text-books.
A standard work is his 'Leçons d'Anatomie comparée,' and, in truth, he is the founder of that kind of comparative anatomy which was brought to such a high state by his pupil, the late Sir Richard Owen. Cuvier discovered the law of 'correlation of growth,' and was the first to apply this law to the reconstruction of animals from fragments: see his monumental work entitled 'Recherches sur les Ossemens fossiles,' 1812.
Cuvier, however, as a strict matter-of-fact man, was incapable of appreciating the speculative conclusions which were drawn by his contemporaries Saint-Hilaire and Lamarck. On the contrary, he firmly stuck to the doctrine of the immutability of species; and, in order to account for the existence of animals whose kind exists no longer, he invented the famous doctrine of successive cataclysms.
Karl Ernst [von Baer] was born in 1792 in Esthonia, studied at Dorpat and then at Würzburg, where Döllinger introduced him to comparative anatomy. For a few years he was a Privat-docent at Berlin; then he went to Königsberg as Professor of Zoology and Embryology. In 1834 he became an Academician at St. Petersburg, where for many years he was occupied with the most varied studies, chiefly geographical and ethnological. The last years of his long, active life he spent in contemplative retirement on his paternal estate, and he died at Dorpat in 1876.
While still at Würzburg he induced his friend Pander, a young man of means, to study the development of the chick; and Pander was the first to start the theory of the germinal layers from which all the organs arise. Baer, however, continued these researches in Königsberg, and after nine years' labour produced his epoch-making work, 'Ueber Entwicklungsgeschichte der Thiere: Beobachtung und Reflexion,' Königsberg, 1828. Nine years later he completed the second volume. He established upon a firm basis the theory of the germinal layers, and by further 'reflexions' arrived at the elucidation of some of the most fundamental laws of biology. For example, in the first volume he made the following prophetic statement: 'Perhaps all animals are alike, and nothing but hollow globes at their earliest developmental beginning. The farther back we trace their development, the more resemblance we find in the most different creatures. And this leads to the question whether at the beginning of their development all animals are essentially alike, and referable to one common ancestral form. Considering that the "germ" (which at a certain stage appears in the shape of a hollow globe or bag) is the undeveloped animal itself, we are not without reason for assuming that the common fundamental form is that of a simple vesicle, from which every animal is evolved, not only theoretically, but historically.'