THE TEETH OF MAMMALS. 277 



dermal appendages lias been sliown at a previous page (see 

 page 2). 



He would indeed be a rash man who ventured to assert 

 that we had recognised all the agencies which are at work 

 in the modelling of animal and vegetable forms ; but it is 

 safe to say that, at the present time, we are acquainted with 

 " natural selection," or " survival of the fittest," an agency 

 by which variations beneficial to their possessors will be 

 preserved and intensified in successive generations; of 

 " sexual selection," which operates principally by enabling 

 those possessed of certain characters to propagate their race, 

 while others less favoured do not get the opportunity of so 

 doing ; of " concomitant variation " between different parts 

 of the body, an agency much more recondite in its opera- 

 tions, but by which agencies affecting one part may second- 

 arily bring about alterations in some other part. 



And operating in the contrary direction, we have a certain, 

 fixity of organisation, so that the power of inheritance is 

 constantly asserting itself by the retention of parts which 

 have become useless, for a time at all events, and by the 

 occasional reappearance of characters which have been lost. 



Allusion has been made to these great biological questions 

 with the view of helping the student to have patience to 

 master descriptions of minute points, of w T hich he does not 

 at the moment sec the bearing, by giving him confidence 

 that there are no characters so trivial but that they may 

 throw very important light upon the remote parentage and 

 the line of descent of the creature under examination. And 

 as a further incentive to painstaking and minute observa- 

 tion, it may be added, that things which are rudimentary, 

 and therefore inconspicuous, are often just the things which 

 happen to teach us most ; for being of no present use, they 

 are not undergoing that rapid change in adaptation to the 

 creature's habits which may be going on in organs which are 

 actively employed. 



