Their ape adding the mansubla is one of the list af at highly developed orien of m the moo.trenes Szivarias and Brides the liver retresentres of the same Can and Aprena teliest of bong birds. Isolated on their land ontinents and free from the competition of higher forms, and espertally from the attacks of carditores these lowly organized and almost defenseless spesies have retained to a marked degree the characterTatics of their remite ancestors. Where the separation of hand and continent has taken place more recently, or where the channel is shallow or narrow, there is greater resemblance between their faunas. Thus, wild animals of Great Britain are quite the same as those of western Europe. The number of species found on islands is usually small as compared with those inhabiting an equal continental area, because the number of ancestral forms which have been carried to the island by currents, wind, and man is likely to be small.

The animals found on high mountain peaks and ranges are distinctly allied to arctic forms. On the northward

retreat of the great ice sheet which, during the last glacial period, covered much of Europe, Asia, and North America, these boreal species were left stranded in, and have since been confined to, regions having arctic characteristics. Species which are closely related to one another are known to inhabit mountain peaks, separated by long stretches of lowland, on which animals of the arctic type could not possibly exist. Their presence can be explained only on the supposition that they are descended from forms which inhabited the entire region during the glacial period and that when the climate became warmer these animals retreated to the cold mountain tops. Many oceanic islands are destitute of batrachians and terrestrial mammals, these animals not having had an independent evolution in these localities, nor being able to make their way out from the mainland. On the other hand, aerial mammals, as bats, are of nearly universal distribution.

It is generally admitted that each species originated in one locality and, by migration, spread into neighboring regions, becoming modified as dispersal brought it into different environments, thus giving rise to variations which ultimately resulted in the development of a number of more or less closely related species.

Such are the main features of the theory of evolution and of Darwin's explanation of the process through variation, heredity, and natural selection. As a subordinate factor should be mentioned his theory of sexual selection, by which is meant that the choice of mates, among the higher animals at least, is largely determined by such physical characteristics as strength, beauty of form, coloration, and vocal powers. Those individuals, for instance, which possessed any of these pleasing characteristics in a higher degree than their companions would be more likely to find mates and to leave de

DODGE'S GEN. ZOÖL.-30

a man and i selection 2.7.2 reting to be accepted by many is the prices by vica en is init teared us to the mportance to be soused to the Darvinian fastır. 12 tal selection. ef regarde zu the mam but not the

è means of modifiation Search for additional means has seen made and la still being prosecuted. Thus conmurutzen la claumed to be a tooling agent in the we and divule of organa, in the adoption of new habits, in the vilection of exironment in the choice of mates. Itslation due to the geographical separation of Indola a ver of species, or to the inability of forms "Winterbreed' piysiological innlation has been suggested at another came of modification. Still another factor, organic selection, has been proposed. It is claimed for this that the adoption of a new habit by an animal wiil lead to the development of structures adapted to the habit, and thus produce changes in specific differences.

The most important addition to the philosophy of organic evolution made since Darwin is Weismann's theory of the continuity of the germ plasm, which maintains, supported by facts of observation, that the essential germinal substance is transmitted from generation to generation through the reproductive cells. Whether or not this material- the bearer of heredity

may be influenced by structural and physiological changes occurring in the species, and thus be transmitted to descendants, is a question which has not yet been definitely answered.

NOTES

1 The complete and elaborate natural history of a single species or limited group is called a Monograph, as Darwin's "Monograph of the Cirripedia." A Memoir is not so formal or exhaustive, giving mainly original investigations of a special subject, as Owen's "Memoir on the Gorilla."

2 Before the time of Linnæus, the ladybug, e.g., was called "the Coccinella with red coleopters having seven black spots." He called it Coccinella septem-punctata.

3 Mondino (1315) and Berenger (1518) of Bologna, and Vesalius of Brussels (1543), were the first anatomists. Circulation of the blood discovered by Harvey, 1616. The lacteals discovered by Asellius, 1622, and the lymphatics by Rudbek, 1650. Willis made the first minute anatomy of the brain and nerves, 1659. The red blood corpuscles were discovered by Swammerdam, 1658. Infusoria first observed by Leeuwenhoek, 1675; the name given by Müller, 1786. Swammerdam was the founder of Entomology, 1675. Comparative anatomy was first cultivated by Perrault, Pecquet, Duverney, and Méry, of the Academy of Paris, the latter part of the seventeenth century. Malpighi, the founder of structural anatomy, was the first to demonstrate the structure of the lungs and skin, 1661. About the same time, Ray and Willoughby first classified fishes on structural grounds. Foraminifers were seen by Beccarius one hundred and fifty years ago; but their true structure was not demonstrated till 1835, by Dujardin. Peyssonel published the first elaborate treatise on Corals, 1727. Haller was the first to distinguish between contractility and sensibility, 1739. White blood corpuscles discovered by Hewson in 1775. Spallanzani was the first to demonstrate the true nature of the digestive process, 1777. Cuvier and Geoffroy, in 1797, proposed the first natural classification of animals. Before that, all invertebrates were divided into insects and worms. Lamarck was the first to study mollusks, 1800; before him, attention was confined to the shell. He separated spiders from insects in 1812. The law of correlation enunciated by Cuvier, 1826. Von Baer was the founder of Embryology, establishing the doctrine omnia ex ovo, 1827; but the first researches in Reproduction were made by Fabricius about 1600, and by Harvey in 1651. Wolff, in the 18th century, was the pioneer in observing the phenomena of Development. Sars first observed alternate generation, 1833. Duméril is considered the

fatter of Herpetology, and wen of Odomology. Schleilen and Schwann pibusbrt ther centrated researches in tel structure, 1841; but Bichat, whi dec 1802 was the fade & Hangy. Protoplasm was discovered by Dawan a 1955, and caled Servde. The name Protoplasma was femaly given to the sony coments of vegetable cells by the German botans. Hig. 1 X C. I 18t The essential identity of the protoplasm Jé plants and animar was irst claimed by Max Schulze in 1861, who thus mace one of the most mp etant generalizations in science. * According t Mr. Darwin, the characters which naturalists consider as showing true afinity between two more species are those which have been inherited from a comme parent; and, in so far. all true classification is genealogical, 14, 1 is at a mere grouping of like with like, but it inclades, Ike descent, the cause of smarty. In the existing state of science a perfect classiable is mpssile, for it involves a perfect knowledge of all animal structure and Le history. As it is, it is only a provisional attempt to express the real order of nature, and it comes as near to it as our laws do in explaining phenomena. It simply states what we now know about comparative anatomy and physilligy. As science grows, its language will become mure precise and its classification more natural.

* The term riges also used to signify that form which presents all the characters of the group most completely. Each genus has its typical species, each order is typical genes, etc. The word is also applied to the specimen on which a new species is funded. A persistent type is one which has continued with very little change through a great range of time. The family of cysters has existed through many geological ages.

• The Corienterata and Echinodermata together make up the Radiata, the old subkingdom of Cuvier. Echinoderma is probably more correct than Echinodermata, but we retain the cld orthography.

7

* Strictly speaking, no individual is independent. Such is the division of labor in a hive, that a single bee, removed from the community, will soon die, for its life is bound up with the whole. An individual repeats the type of its kingdom, branch, class, order, family, genus, and species, through its whole line of descent.

* These definitions of the various groups are mainly taken from Agassiz. They are not practically very useful, as they are not used by working naturalists. The kind and degree of difference entitling a group to a particular rank varies greatly with the naturalist, and the part of the animal kingdom where the group is found. Some families of insects are separated by gaps less than those which divide genera of mammals.

• The millepore coral, so abundant in the West Indian Sea, is the work of hydroids. The surface is nearly smooth, with minute punctures. Gegenbaur, Haeckel, and others hold that the acalephs have no body cavity at all, the internal system of canals being homologous with the intestinal cavity of other animals.