Physiologists and philosophers have made many attempts to define 'life,' but in most cases in aiming at absolute generality they have been vague and uninstructive. Thus De Blainville defined it as 'The twofold internal movement of composition and decomposition, at once general and continuous'; while Herbert Spencer's latest definition was 'Life is the continuous adjustment of internal relations to external relations.' But neither of these is sufficiently precise, explanatory, or distinctive, and they might almost be applied to the changes occurring in a sun or planet, or to the elevation and gradual formation of a continent. One of the oldest definitions, that of Aristotle, seems to come nearer the mark: 'Life is the assemblage of the operations of nutrition, growth, and destruction.' But these definitions of 'life' are unsatisfactory, because they apply to an abstract idea rather than to the actual living organism. The marvel and mystery of life, as we know it, resides in the body which manifests it, and this living body the definitions ignore. The essential points in the living body, as seen in its higher developments, are, first, that it consists throughout of highly complex but very unstable forms of matter, every particle of which is in a continual state of growth or decay; that it absorbs or appropriates dead matter from without; takes this matter into the interior of its body; acts upon it mechanically and chemically, rejecting what is useless or hurtful; and so transforming the remainder as to renew every atom of its own structure internal and external, at the same time throwing off, particle by particle, all the worn-out or dead portions of its own substance. Secondly, in order to be able to do all this, its whole body is permeated throughout by branching vessels or porous tissues, by which liquids and gases can reach every part and carry on the various processes of nutrition and excretion above referred to. As Professor Burdon Sanderson well puts it: 'The most distinctive peculiarity of living matter as compared with non-living is, that it is ever changing while ever the same.' And these changes are the more remarkable because they are accompanied, and even produced, by a very large amount of mechanical work--in animals by means of their normal activities in search of food, in assimilating that food, in continually renewing and building up their whole organism, and in many other ways; in plants by building up their structure, which often involves raising tons of material high into the air, as in forest trees. As a recent writer puts it: 'The most prominent, and perhaps the most fundamental, phenomenon of life is what may be described as the _Energy Traffic_ or the function of _trading in energy_. The chief physical function of living matter seems to consist in absorbing energy, storing it in a higher potential state, and afterwards partially expending it in the kinetic or active form.'[11]