AIM AND PROGRESS OF PHYSICAL SCIENCE. 341 



dual modification of species after a succession of generations, we 

 know that a single individual may adapt itself, or become 

 accustomed, in a certain degree, to the circumstances under which 

 it has to live ; and that even during the single life of au indi- 

 vidual a distinct progress towards a higher development of 

 organic adaptability may be attained. And it is more especially 

 in those forms of organic life where the adaptability in structure 

 has reached the highest grade and excited the greatest admiration, 

 namely, in the region of mental perception, that, as the latest 

 results of physiology teach us, this individual adaptation plays 

 a most prominent part. 



Who has not marvelled at the fidelity and accuracy of the 

 information which our senses convey to us from the surround- 

 ing world, more especially those of the far-reaching eye ? The 

 information so gained furnishes the premisses for the conclusions 

 which we come to, the acts that we perform ; and unless our 

 senses convey to us correct impressions, we cannot expect to act 

 accurately, so that results shall correspond with our expectations. 

 By the success or failure of our acts we again and again test 

 the truth of the information with which our senses supply us, 

 and experience, after millions of repetitions, shows ns that this 

 fidelity is exceedingly great, in fact, almost free from exceptions. 

 At all events, these exceptions, the so-called illusions of the 

 senses, are rare, and are only brought about by very special and 

 unusual circumstances. 



Whenever we stretch forth the hand to lay hold of some- 

 thing, or advance the foot to step upon some object, we must 

 first form an accurate optical image of the position of the 

 object to be touched, its form, distance, <fec., or we shall fail. 

 The certainty and accuracy of our perception by the senses 

 must at least equal the certainty and accuracy which our actions 

 have attained after long practice j and the belief, therefore, in 

 the trustworthiness of our senses is no blind belief, but one, the 

 accuracy of which has been tested and verified again and again 

 by numberless experiments. 



Were this harmony between the perceptions through the 

 senses and the objects causing them, in other words, this basia 



