On Animal Insfinct : in its Relation to the Mind of Man. 337 



It is the unity wliich exists between the living organism and the ele- 

 ments around it which renders that organism the appropriate channel 

 of communication with the external world, and a faithful interpreter 

 of its signs. And this the organism is, not only by virtue of its sub- 

 stance and composition, but also and especially by virtue of its adjusted 

 structures. All the organs of sense discharge their functions in virtue 

 of a mechanical adjustment between the structure of the organ and 

 the particular form of external force which it is intended to receive 

 and to transmit. How fine those adjustments are can best be under- 

 stood when we remember that the retina of the eye is a machine which 

 measures and distinguishes between vibrations which are now known 

 to differ from each by only a few raillionths of an inch. Yet this 

 amount of difference is recorded and made instantly appreciable in the 

 sensations of color by the adjusted mechanism of the eye. Another 

 adjustment, precisely the same in principle, between the vibrations of 

 sovmd and the structure of the ear, enables those vibrations to be sira- 

 , ilarly distinguished in another special form of the manifold language 

 of sensation. And so of all the organs of sense — they all perform their 

 work in virtue of that purely mechanical adjustment which places 

 them in a given relation to certain selected manifestations of external 

 force, and these they faithfully transmit according to a code of signals 

 the nature of which is one of the primary mysteries of life, but the 

 truthfulness of which is at the same time one of the most certain of its 

 facts. 



For it is upon this truthfulness — that is to say, upon a close and 

 efficient correspondence between the impressions of sense and the reali- 

 ties of external nature — that the success of every organism depends in 

 the battle of life. And all life involves a battle, for though it comes 

 to each animal without effort of its own, it cannot be maintained with- 

 out individual exertion. That exertion may be of the simplest kind, 

 nothing more than the rhythmic action of a muscle contracting and 

 exj)anding so as to receive into a sac such substances as currents of 

 ■water may bring along with them ; or it may be the more complex 

 action required to make or induce the very currents which are to bring 

 *the food ; or it may be the much more complex exertions required in ail 

 active locomotion for the pursuit and capture of prey : all these forms 

 of exertion exist, and are all required in endless variety in the animal 

 world. And throughout the whole of this vast series the very life of 

 every creature depends on the perfect correspondence which exists 

 between its sense-impressions and those realities of the external world 

 which are specially related to them. There is therefore no conception 

 of the mind Avhich rests on a broader basis of experience than that 



