MODERN PHILOSOPHICAL BIOLOGY. 711 



a less stable to a more stable equilibrium. The atoms of the organic 

 substance lose part of their latent motion, which is manifested exter- 

 nally under the form of heat, electricity, light, nervous force, or me- 

 chanical motion, according to circumstances. Be the cause which 

 produces these clianges necessary or not, they are, of necessity, accom- 

 panied by a disengagement of force ; and we can affirm of any force 

 whatever expended by an organ of a living being, that it is the 

 equivalent of a force acting from without upon that being. This is a 

 consequence of the law of the persistence of force, and it may be pre- 

 sented under two forms : First, in order that a certain amount of force 

 may be expended by a living being, there must have taken place, 

 within that being, a transformation, by decomposition, of a quantity 

 of organic substance capable of holding that force in the latent stale ; 

 and, secondly, there can be no transformation of organic matter 

 holding force in the latent state, without an expenditure of force 

 wliich shall manifest itself in some shape externally. 



In general terms, what we have to consider in living things is, 

 first, a substance of special composition, and then expenditures of 

 force by that substance ; and this, too, is what we have in general 

 terms to consider, in non-living things. The former are distinguished 

 from the latter by the fact that the changes which constitute their 

 history are heterogeneous; that they form many series which are 

 simultaneous, correlative, held together by a tie of mutual dependence, 

 the result being a high degree of complexity, a phenomenon belonging 

 to one series haA^ing antecedents and consequents in other simultaneous 

 series; and above all, that these changes form clearly-defined combina- 

 tions. This ensemble of characteristics not only enables us to dis- 

 tinguish living from non-living things, but also to distinguish between 

 living things themselves and to class them according to their degree of 

 life. Thus a thing stands all the higher in the vital scale in jtropor-^ 

 tion as, from the beginning to the end of its vital manifestations, it 

 exhibits a larger number of successive and simultaneous changes, and 

 as these changes are more heterogeneous and more closely linked to- 

 gether, and in more definite relations to one another. Between the 

 lowest animals, rhizopods, planaria, etc., and the highest, the birds of 

 prey, manmalia, carnivora, man, there is an enormous dissimilarity ; 

 still the definition applies to them all, and serves to define the difler- 

 ence which separates them, as also the difierence of the numerous 

 species lying between these extremes of the animal series. 



Though this definition is a good one, inasmuch as it applies to all 

 living things, and to them alone, nevertheless it is defective in that 

 it omits the most distinctive peculiarity, namely, the element known 

 as activity, in other words, those operations whereby living beings 

 adapt themselves to their conditions of life. The definition should 

 include the general relations of the living thing to its environment. 

 The environment, too, has its successive and correlative changes 



