have been set apart to perform a certain particular round of duties, while other cells have been set aside for other work. For example, the protoplasm of certain cells has become very contractile and forms the muscles; in certain other cells the protoplasm has become highly irritable and responsive, and makes the nervous tissues; and so on for the other groups. This physiological division of labor has led to the different tissues.
Plants show very nicely the gradations between the single-celled and the many-celled condition. There are first linear aggregates, in which the cells are united end to end in a single row. The next step is groups of cells arranged in a single layer to form an expanded surface; and finally the combination of cells into a solid mass having length, breadth and thickness.
In biology, animals and plants are considered from a variety of view-points: as to their Structure, or the way in which they are constructed (see Anatomy); as to their Development, or the stages through which they pass from the egg or seed to the adult (see Development); as to their Physiology, or the uses of the different organs and the changes that are taking place in the protoplasm of the tissues; as to their Distribution. Biology is, therefore, a complex science, and is the result of the concurrent progress in all these departments. It may be likened to a great stream into which a number of smaller streams have united to make the main one, and it contains mixed together the product of all.
The main divisions of biology are, of course, greatly subdivided; for example, under structure, we might consider animals and plants in reference to their surroundings, and show that the structural peculiarities are the result of responses to the surrounding conditions, and we might further show how likeness in structure indicates relationship, and is the basis upon which animals and plants are classified or arranged into systematic groups. Moreover, development and physiology are very extensive branches, and must be divided into smaller topics for practical consideration. In reference to the distribution of animals and plants, it must be said that it takes two directions: first, their geographical distribution, and, secondly, their distribution in time. The first will be clear without further statement, but the second requires a word. We know that there are entombed in the rocks countless numbers of animals and plants that lived centuries ago and became extinct. The succession of life in the rocks is very interesting, beginning as it does with the lowest forms, in the earliest formed rocks, and passing to the higher ones in the later formed rocks. In this succession of stages we can read the past history of life on the earth, and this has helped greatly in establishing the doctrine of organic evolution.
It is an indefinite line that separates biology from botany and zoölogy. Modern botany and zoölogy embrace all that is known about plants and animals respectively, but the plant kingdom and the animal kingdom are considered separately. In biology the facts are approached from a different standpoint, and the emphasis is differently placed. The phenomena of life are brought into union in both animals and plants, and the attention is especially directed to the activities of protoplasm, and its responses to surrounding conditions. General biology is a term in common use to indicate the consideration of certain general facts about animals and plants. It is recognized as a distinct branch, and frequently studies of these general topics are made to precede studies that are mainly botanical on the one hand, or mainly zoölogical on the other.
These facts should make clear how the department of biology arose and what it is about; but before leaving the subject we should at least glance at its 19th century features. The three things that most distinctly mark biological advance during the 19th century are: (1) The cell-doctrine (which see),—the discovery of the fact that, with the exception of unicellular forms, plants and animals are composed of groups of cells, and moreover, that they all begin their existence as eggs or ovules, in the single-cell condition. (2) The discovery of protoplasm (which see), and the recognition of the rôle it everywhere plays in animal and plant life. (3) The doctrine of organic evolution, or the discovery of the genealogy of animals and plants.
There are in addition other things to be mentioned: The great extension of knowledge in reference to microbes and bacteria (which see) has been characteristic. Advances in this direction have led to the discovery of the nature of fermentation, of decay, to the germ theory of disease, etc.; and have also brought in their train an unusual number of practical applications: antiseptic surgery, the canning of fruits and meats, infecting insects with disease to stop the ravages of the injurious kinds; and also the protection of silk-worm culture, etc.
The growth of information regarding the development of animals and plants has been very great, and has been turned to account in reading the past history of life.
The question of the spontaneous origin of life was revived in 1858. That is the belief that the simplest microscopic forms of life are sometimes formed, spontaneously, from lifeless matter. But it was
