Division of Physiological Labour. Conditions of Existence of Living Matter.
When the different forms of life are compared together as physiological machines, they are found to differ as machines of humans construction do. In the lower forms, the mechanism, though perfectly well adapted to do the work for which it is required, is rough, simple and weak; while, in the higher, it is finished, complicated, and powerful. Considered as machines, there is the same sort of difference between a polype and a horse as there is between a distaff and a spinning-jenny. In the progress from the lower to higher organism, there is a gradual differentiation of organs and of functions. Each function is separated into many parts, which are severally entrusted to distinct organs. To use the striking phrase of Milne-Edwards, in passing from low to high organisms, there is a division of physiological labour. And exactly the same process is observable in the development of any of the higher organisms, so that, physiologically, as well as morphologically, development is a process from the general to the special.
Thus far, the physiological activities of living matter have been considered in themselves, and without reference to anything that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it; and the study of the influence of the "conditions of existence" thus determined constitutes a most important part of Physiology.
The sustentative functions, for example, can only be exerted under certain conditions of temperature, pressure, and light, in certain media, and with supplies of particular kinds of nutritive matter; the sufficiency of which supplies, again, is greatly influenced by the competition of other organisms, which, striving to satisfy the same needs, give rise to the passive "struggle for existence". The exercise of the correlative functions is influenced by similar conditions, and by the direct conflict with other extensive modifications, dependent partly upon what are commonly called external conditions, and partly upon wholly unknown agencies.
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