[81] THE EVOLUTION OF THE FINS OF FISHES. 1061 



reflex actions of higher animals are finally reducible to molecular dia- 

 turbauces and more or less spasmodic exhibitions of contractions of 

 aggregates of plasmic bodies in response to nervous stimuli or impulses 

 generated at the periphery as sensations and finally sent back from the 

 nervous centers as motor impulses of the organism which regulate and 

 w-ordinate the contractions exhibited in the course of vital work in the 

 struggle for existence. All of the energy so exhibited is not dissipated 

 in the visible movements of the animal, but some of it is unquestion- 

 ably consumed in inducing morphological alterations or adjustments, in 

 maintaining a physiological equilibrium, in other words; or, as hap- 

 pens in a mechanical device, some force is consumed in overcoming fric- 

 tion. This kind of an equivalency between a certain residual part of 

 the energy expended by an organism and the energy needed to be used 

 in the acquirement of new organs is clearly correlative aud conservative. 

 The acquisition of new organs may therefore very probably be viewed 

 as a problem of phj^siological dynamics, in which a certain equivalency 

 between the work done, the bulk of the living mass, the distance 

 through which movement is made, and the duration of the eifort exerted 

 will be taken into account so as to get at a residual element of potential 

 and dissipated energy used up in maintaining and readjusting the or- 

 ganic aggregate to new conditions. The development of new organs is 

 evidently a progressive process, as our study of the evolution of fishes' 

 tails has shown, for it is hardly possible to conceive that the ascending 

 order of differentiation indicated by ontology and palaeontology can be 

 in error, since that order is approximately as follows: archicercy, lopho- 

 cercy, diphycercy, heterocercy, homocercy, and gephyrocercy. 



When an aggregate of plasmic units or an organ, such as a muscle, is 

 stimulated to contract for the first time in response to a new or extraor- 

 dinary effort, increased waste follows, which must be made good by 

 increased nutritional activity in that part. More blood is therefore sent 

 to such parts momentarily, the remarkable vaso-motor system of nerves 

 being involved in effecting this in higher organisms. In simpler organ- 

 isms, however, no provision of vessels and vaso-motor nerves exists, 

 and the acquisition of pabulum may be effected in some cases by its 

 mere transfer iirom cell to cell, as is the case in many processes of em- 

 bryonic growth. The very remoteness of the peripheral plastids from 

 the nutritive and respiratory centers in higher organisms has led to the 

 evolution of the vaso-motor and vascular systems, so as to provide for 

 the reflex exhibition of the needs of remotely-situated groups of plastids 

 when extraordinary exhibitions of effort are in progress in such groups, 

 and thus enable their immediate wants to be supplied. Such systems 

 are beyond the control of the will as expressed and exerted under the 

 guidance of the rational faculties, aud they minister to the demands of 

 the aggregate of the wills, so to speak, of aggregates of remotely- sit- 

 uated plastids calling in this way for oxygen and food. This cellular 

 volition is the exhibition of the low protozoan grade of reflex action 



