MODERN PHILOSOPHICAL BIOLOGY. 715 



lications of the aggregate to which they belong. Tliey undergo tlie 

 influence of the environment, though indirectly, through this aggre- 

 gate. Their modifications are new directions, amplitudes of new vibra- 

 tions, which place them in equilibrium with the forces which the en- 

 semble of the aggregate, as modified by the environment, brings to 

 bear upon them. These moditications endure as long as equilibrium 

 endures, and are ever transmitted to the new units which spring from 

 the former ones, until, on the equilibrium being disturbed, a new 

 breaking-up of the existing relations necessitates others. 



The hypothesis of physiological units is a necessity, not only in 

 order to fill up tlie gap which separates the highest products of or- 

 ganic chemistry from those irreducible elements revealed by the mi- 

 croscope which we call morphological elements, but also in order to 

 furnish a substratum for the positive property which serves to account 

 for the great facts of biology, and to refer them, by formulae expressed 

 in terms of mechanics, to first principles. 



Let us now consider the great facts of biology. 



The growth of an organism is an operation essentially like the 

 growth of a crystal. " Around a plant there exist certain elements 

 that are like the elements which form its substance ; and its increase 

 of size is efiected by continually integrating those surrounding like 

 elements with itself. !Nor does the animal fundamentally differ in this 

 respect from the plant or the crystal. Its food is a portion of the en- 

 vironing matter that contains some compound atoms like some of the 

 compound atoms constituting its tissues ; and, either through simple 

 imbibition or through digestion, the animal eventually integrates with 

 itself units like those of which it is built up, and leaves behind unlike 

 units." 



Organic growth differs from inorganic in this, that it has limits. 

 All conditions remaining the same (a proviso that must always be 

 made in biology), and the quantity of integrated substance not vary- 

 ing, we find that, by the principle of the persistence of force, the 

 growth of the living being must depend on the expenditure. The only 

 portion of the integrated substance that can serve for growth is the 

 unexpended residue, the excess of nutrition over expenditure a quan- 

 tity which is essentially variable, and which transfers its variations 

 to the growth, limiting it and diminishing it more or less rapidly from 

 the moment when the body of the living thing has attained its full 

 development. Experience shows that the limit of growth is fixed for 

 those organisms which have large expenditure, and that for those 

 which have hardly any expenditure this limit gradually recedes ; of 

 tliis the crocodile is an instance. But there is another element which 

 must be taken into account, namely, that the definitive volume of an 

 organism, being the sum of its initial volume and of its successive 

 increments, must depend upon the initial volume. The definitive vol- 



