164 REFLEX ACTION AND 



same organism, gradually developing througli successive 

 generations or stages of one life history.* 



The doctrine of ' Inherited Acquisition,' to the enuncia- 

 tion and development of which we are so largely indehted 

 to Herbert Spencer, explains, therefore, how it is that 

 young organisms, only just arrived at maturity, are often 

 better adapted, in some respects, to their surroundings 

 than were their predecessors, near or remote, at a corre- 

 sponding age. Consequently, if during their lifetime 

 again, or during that of their descendants, some further 

 modes of impressibility (with corresponding powers of 

 discrimination) become possible either in old or in new 

 directions ; and if simultaneously there arises some new 

 or altered capacity for acting in response to these new 

 impressions, it will not be difficult for the reader to 

 understand that this would constitute one important mode 

 in which the nervous system slowly develops and becomes 

 more complex. 



Thus it is that habitual or often recurring stimuli of 

 new kinds are presumed to be constantly leaving their 

 traces in the plastic tissues of lower organisms, and 

 inducing such structui'al modifications in them as tend not 

 only to make the recurrence of similar impressions more 

 easy, but also to render the reception and recognition 

 of new impressions more possible. 



Most of us must be familiar with the fact that by the 

 concentration of Attention in certain directions, aided by 

 voluntary efforts, we are capable of increasing our powers 

 of Discrimination in the range of either of the senses, and 



* The many influences capable of accelerating or retarding this 

 kind of race development cannot here be even enumerated. Suffice 

 it to say that some of the principal of them have been described 

 and copiously illustrated by Mr. Darwin in his works on " The 

 Origin of Species," and on " Sexual Selection." 



