48 BIOLOGICAL LECTURES. 



relation to one another and to the earth's center, in virtue of 

 the action of the forces of cohesion, friction, gravitation, etc. 

 The different regions of such an aggregate now adjust them- 

 selves to the surroundings in such a way that nearly constant 

 effects of light, heat, etc., begin to control or affect the func- 

 tions of such an aggregate dynamically through its metabolism. 

 Function, thus conditioned, asserts itself under the stress of 

 mechanical adaptation or adjustment that becomes increasingly 

 complex with every advance in ontogeny. Every step in on- 

 togeny becomes mechanically adaptive and determinative of 

 the next. It is thus only that we can understand the wonder- 

 ful molecular sorting process that goes on in ontogeny, for 

 which others have invoked infinite multitudes of needless 

 "gemmules," "biophors" and "determinants." 



It is the whole organism that develops in continuity or coor- 

 dination ; not its nuclei, centrosomes, and asters only. The 

 whole organism, molecularly considered, is as fixed and immu- 

 table, within variable limits, as a crystal. Its development, 

 moreover, becomes intelligible only if we contemplate its 

 ontogeny somewhat as we would the growth of a crystal, with 

 the additional supposition that its growth is not conditioned 

 by forces operating along straight lines having a constant 

 angular divergence as in the latter. On the contrary, living 

 matter is capable of developing curved bounding surfaces in 

 consequence of the permanently mobile nature and cohesion of 

 its molecules, that, as a complex dynamical mechanism, can 

 operate so as to tell off the tale of its transformation in but 

 one way, in consequence of the order and way in which the 

 energy of its constituent molecules is set free during ontogeny. 

 Upon the completion of ontogeny a phase is reached in which 

 the income and outgo of metabolism is in equilibrium. The 

 duration of life depends upon the length of time that this equi- 

 librium can be maintained without fatal impairment of the 

 harmonious operation of its mechanism under the stress of the 

 dynamical conditions of life. This may be considered the cause 

 of death, so that the length of the life of the individual is 

 determined by the possible number of harmonious molecular 

 transformations of which its plasma is capable as a mech- 

 anism. 



