284 GENERAL REMARKS. 



one form after another coming into being successively 

 would each introduce some novel modifications, ac- 

 cording to its place in time and the structural series. 

 These modifications being inherited at earlier stages 

 in descendants than those in which they originated in 

 the ancestral forms, would crowd upon the character- 

 istics already fixed by heredity in the growth of the 

 young. By and by, as characteristics accumulated, it 

 would become not only inconvenient to repeat all the 

 characteristics of its ancestors, but it would be a. phys- 

 ical impossibility for any individual to reproduce them 

 all in the same succession in which they had arisen ; 

 life would not be long enough nor vital powers strong 

 enough to accomplish such a process. Nature pro- 

 vides for such emergencies by a law of replacement ; 

 and as stated above, when a part or characteristic 

 becomes useless, if it stand in the way of the devel- 

 opment of other parts or other characteristics of the 

 same part, it is replaced to a greater or less degree by 

 the newer and more useful modifications. This is the 

 rule so far as relates to an ordinary normal series of 

 forms when such a series can be traced with abundant 

 materials through a sufficiently long period of geologic 

 time, as has been repeatedly shown by Cope and one 

 of the authors. Made confident by such experiences 

 we do not hesitate to apply it to the insects where 

 positive evidence of this sort is not yet forthcoming. 



If this be correct, it is evident for example that the 

 sucking-tube and other correlative internal modifica- 

 tions originated in the pupal or adult stages of the 

 primitive Hemipteron, then became fixed in the organ- 

 ization of the order, and are now inherited at an early 



