524 THE POPULAR SCIENCE MONTHLY. 



first signs it gives of its activity are movements comparable with 

 those of the amoeba. Thus, without effort, we find on different 

 sides life freed from form. We comprehend that it is not essen- 

 tially and fatally bound to form. A body may be living and still 

 have no definite figure. Here the problem is suggested, whether 

 a liquid, a bodily humor, can be living. Is the blood living, like 

 the substance of the nerves or the flesh of the muscles ? It is a 

 deep question and has not yet been answered. At any rate, science 

 has been led for a long time to look for the characteristic of life 

 somewhere else than in form. 



The Aristotelians saw a movement in what we call life ; and 

 they gave that name to every change of state of natural bodies as 

 well as to their translation proper in space. Aristotle's treatise 

 on the Soul characterizes life by the three facts of its nourishing 

 itself, developing, and perishing. Growth and decline are changes, 

 and consequently movements ; and, as we always see them closely 

 connected with the feeding of the plant as well as of the animal, 

 we find the act of feeding definitively at the basis of the movement 

 which is life. Moreover, do we not see during growth the parts 

 of which the creatures are composed changing places relatively to 

 one another ? Have we not here a clear, absolute distinction from 

 the increase of mineral bodies ? 



There are, however, some parts in animals which grow by a 

 simple constant accretion of superadded new particles ; such as 

 the shells of mollusks, even when they are covered by the flesh, 

 like cuttlefish bone. But these formations, although derived from 

 the organism, are not themselves living. They bear, if we may 

 say so, the stamp and seal of life so far that we can recognize 

 them as a product of it, but no further ; and if they grow, it is as 

 crystals do. 



Thomas Aquinas, following Aristotle, gave life the most exact 

 definition that could be made with the knowledge of his time. 

 It is almost as satisfactory for us, for we, too, define life in the 

 same terms. It is a movement, but still not one of the appar- 

 ent though intimate movements to which the Christian encyclo- 

 pedist alludes. It is a molecular movement that escapes our eyes, 

 in the interior of the being, and is revealed to our senses only by 

 its results. 



The movement that constitutes life is an intimate, profound, in- 

 visible, incessant movement, at once of combination and of decom- 

 position. Living matter is incessantly born and incessantly dying, 

 being formed and suffering destruction all at the same time. 



All liquid or gaseous bodies coming in contact with a living 

 substance and soluble by it, penetrate it, mingle with it, and then, 

 carried on in the whirl, cease for the most part to be themselves, 

 are transformed, enter into new combinations that did not exist 



