In all animals which consist of cell-aggregates, the cells of which the embryo is at first composed arninge themselves by the splitting, or by a process of invagination, of the blastoclerra into two layers, the epiblast, and the hypoblast, between which a third intermediate layer—the niesobtast—appears, and each layer gives rise to a definite group of organs iu the adult. In the rertebrata the epiblast gives rise to the cerebrospinal axis, and the epidermis acid its derivatives; the hypoblast to the epithelium of the alimentary canal and its derivatives; and the mesoblast to all the intermediate structures. The ten dency of late research is to prove that the several layers of the germ evolve analogous organs in invertebrate animals, and to indicate the possibility of tracing the several germ layers bark to the blastomercs of the yolk, from the subdivision of which they all pro ceed.
It may be conceived that all forms of life should have presented nearly the same dif ferentiation of structure, and should have differed from one another by superficial char acteristics, each form passing by insensible gradation into those most like it. In such case, taxonotny (the classification of morphological facts) must have been confined to forming an arrangement representing the serial gradation of these forms in nature. ot it may be conceived that living beings should have differed as widely in structure as they really do, but that the interval between any two extreme forms should have been filled up by an unbroken series of gradations; in which case classification could effect the formation of series only, the strict definition of groups being as impossible as in the former case. But, in fact, living beings differ widely, not only in structure but in the modes in which the differences are brought about; and the intervals between extreme forms are not filled up in the world by complete series of gradations. Hence living beings are, to a great 'extent, susceptible of classification into groups, the members of each group resembling one another, and differing from all the rest by definite pecul iarities. No two living beings are exactly alike, but among endless diversities some stantly resemble one another so.closely that it is impossible to draw a line of demarkation between them, while they differ only in such characteristics as are associated with sex. These constitute a morphological species; while different morphologieal spce:'es are defined by constant characteristics that are not merely sexual. Generic groups thus con= stituted may be arranged into families, orders, classes, etc.
DISTRIAUTION.—Living beings are different in different zones of the earth and in different heights above the sea, or in different climates; and the same is true of living things in the sea. And places that differ in longitude though not differing in climate, may have different animals and plants. Certain areas are inhabited by animal or vege table groups that are not found elsewhere. Such areas are denominated prorinees of dip r•ribution. Such areas have no common agreements, either in extent, boundaries, or phys ical features. Indeed, there are no phenomena in nature more capricious and arbitrary than the distribution of living things, The revelations of geology give us an Idea of the distribution of long extinct species of animal and vegetable life, and we find that entirely different life is now found where these creatures once existed; and the further we go back the wider the differences. In any locality the succession of living forms may appear
to be interrupted by numerous breaks; but the tendency of pahcontological investigation is to show that these breaks are only apparent. It is evident, both with regard to ani mals and plants, that the changes in the living population of the earth which bare taken place during its history, have been effected not by the. sudden displacement of ono set of living beings by another but by a process of gradual introduction of new species and the extinction of older forms. In all parts of the globe in which fossiliferous rocks have been examined, the successive terms of the series of living forms have succeeded each other in a uniform way.
III. PuysioLoov.—The activities of living matter are called its functions; and those functions, though widely varied, may be arranged in three categories. They are: 1. Functions that affect the material composition of the body and determine its mass, which is the balance of the processes of waste on one hand, and those of assimilation on the other. 2. Functions which subserve the process of reproduction, which is essentially the detachment of a part endowed with the power of developing into an independent whole. 3. Functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its part's, or as a whole, becomes a source of .molecular motion. These categories are, 1, sustenIatire; 2, gaiei-atire; 3, co•latire functions. Of the three classes of functions the first two are invariably present in living beings. Some of the lower fungi do not possess the power of changing the form, or the correlative functions. In most of the lower plants,. however, and in all known animals, the body either constantly or temporarily Changes'its form, either with or without the application of a special stimulus, and therebv changes the relations of its parts to one another, and of the whole to other bodies. 'the higher animals produce such changes by means of a special tissue called nerve; motion.on a large scale is effected by another tissue, muscle; and the organism is brought into relation with surrounding things by another special tissue—that of the SLIISOT.71 MalIS, by means of which the forces exerted by surrounding bodies are transmuted into affections of nerve. In the lowest forins of life the functions that have been entunerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body; and the same is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated cell which constitutes the starting point of their development. But the first, process in that development is the division of the germ into a number of morphological units or blastorneres, which eventually- give rise to cells; and 'as each of these possesses the.same physiological fungi ions as the germ itself, it follows that each morphological unit, is also a physiological unit, and the multieellular mass is strictly a compound organism made up of a multitude of physiologically independent cells. The physiological activities manifested by the complex whole represent the sum, or rather the resultant, of the separate and independent physiological activities resident in each of the simpler constituents of the whole.