enemies. In the next year another brood is carefully reared, and so on for the few years of the robin's life. Under normal conditions in any given locality the number of individuals of a certain species of animal remains about the same. The fish which produces tens of thousands of eggs and the bird which reproduces half a dozen eggs a year maintain equally well their numbers. In one case a few survive of many born; in the other many (relatively) survive of the few born; in both cases the species is effectively maintained. In general, no agency for the perpetuation of the species is so effective as that of care for the young. CHAPTER IV FUNCTION AND STRUCTURE 37. Organs and functions.-An animal does certain things. which are necessary to life. It eats and digests food, it breathes in air and takes oxygen from it and breathes out carbonic-acid gas; it feels and has other sensations; it produces offspring, thus reproducing itself. These things are done by the simplest animals as well as by the complex animals. But while with the simplest animals the whole body (which is but a single cell) takes part in doing each of these things, among the complex animals only a part of the body is concerned with any one of these things. Only a part of the body has to do with the taking in of oxygen. Another part has to do with the digestion of food, and another with the business of locomotion. These parts of the body, as we know, differ from each other, and they differ because they have different things to do. These different parts are called organs of the body, and the things they do are called their functions. The nostrils, tracheæ, and lungs are the organs which have for function the process of respiration. The legs of a cat are the organs which. perform for it the function of locomotion. The structure of one of the higher animals is complex because the body is made up of many distinct organs having distinct functions. The things done by one of the complex animals are many; around each of the principal functions or necessary processes, as a center, are grouped many minor accessory functions, all helping to make more successful the accom plishment of the principal functions. While many of the lower animals have no eyes and no ears, and trust to more primitive means to discover food or avoid enemies, the higher animals have extraordinarily complex organs for seeing and hearing, two functions which are accessory only to such a principal function as food-taking. 38. Differentiation of structure.-We have seen, in our study of the slightly complex animals, how the body becomes more and more complex in proportion to the degree in which the different life processes are divided or assigned to different parts of it for performance. With the gradually increasing division of labor the body becomes less homogeneous in structure; a differentiation of structure becomes apparent and gradually increases. The extent of the division of labor and the extent of the differentiation of structure, or division of the body into distinct and different parts and organs, go hand in hand. An animal in which the division of labor is carried to an extreme is an animal in which complexity of structure is extreme. 39. Anatomy and physiology.-Zoology, or the study of animals, is divided for convenience into several branches or phases. The study of the classification of animals is called systematic zoology; the study of the development of animals from their beginning as a single cell to the time of their birth is called animal embryology; the study of the structure of animals is called animal anatomy, and the study of the performance of their life processes or functions is called physiology. Because the whole field of zoology is so great, some zoologists limit themselves exclusively to one of these phases of zoological study, and those who do not so definitely limit their study, at least give their special attention to a single phase, although all try to keep in touch with the state of knowledge in other phases. In earlier days the study of the anatomy of animals and of their physiology were held to be two very distinct lines of investigation, and the anatomists paid little attention to physiology and the physiologists little to anatomy. But we have seen how inseparably linked are structure and function. The structure of an animal is as it is because of the work it has to do, and the functions of an animal are performed as they are performed because of the special structural condition of the organs which perform them. The study of the anatomy and the study of the physiology of animals can not be separated. To understand aright the structure of an animal it is necessary to know to what use the structure is put; to understand aright the processes of an animal it is necessary to know the structure on which the performance of the processes depends. 40. The animal body a machine.-The body of an animal may be well compared with some machine like a locomotive engine. Indeed, the animal body is a machine. It is a machine composed of many parts, each part doing some particular kind of work for which a particular kind of structure fits it; and all the parts are dependent on each other and work together for the accomplishment of the total business of the machine. The locomotive must be provided with fuel, such as coal or wood or other readily combustible substance, the consumption of which furnishes the force or energy of the machine. The animal body must be provided with fuel, which is called food, which furnishes similarly the energy of the animal. Oxygen must be provided for the combustion of the fuel in the locomotive and the food in the body. The locomotive is composed of special parts: the firebox for the reception and combustion of fuel; the steam pipes for the carriage of steam; the wheels for locomotion; the smoke stack for throwing off of waste. The animal body is similarly composed of parts: the alimentary canal for the reception and assimilation of food; the excretory organs for the throwing off of waste matter; the arteries and veins for the carriage of the oxygen and food-holding blood; the legs or wings. for locomotion. The locomotive is an inorganic machine; the animal is an organic machine. There is a great and real difference between an organism, a living animal, and a locomotive, an inorganic structure. But for a good understanding of the relation between function and structure, and of the composition of the body of the complex animals, the comparison of the animal and locomotive is very instructive. 41. The specialization of organs.-The organ for the performance of some definite function in one of the higher animals may be very complex. The corresponding organ in one of the lower animals for the performance of the same function may be comparatively simple. For example, the organ for the digestion of food is, in the case of the polyp, a simple cylindrical cavity in the body into which food enters through a large opening at the apical or free end of the body. The digestive organ of a cow is a long coiled tube, comprising many regions of distinct structural and physiological character and altogether extremely complicated. An organ in simple or primitive condition is said to be generalized; in complex or highly modified condition it is said to be specialized. That is, an organ may be modified and complexly developed to perform its function in a special way, in a way differing in many particulars from the way the corresponding organ in some other animal performs the same general function. The specialization of organs, or their modification to perform their functions in special ways, is what makes animal bodies complex, for specialization is almost always in the line of complexity. Later we shall see more clearly how specialization is brought about. For the present we may study one of the more important organs of the animal body for the sake of having concrete examples of some of the general statements made in this discussion of function and structure. 42. The alimentary canal.-The organ which has to do with the taking and digesting of food is called the ali |