424 KECAriTULATION. Chap. XIV. 



have already varied since they branched off from a common 

 progenitor in certain characters, by which they have come to 

 be specifically distinct from each other ; therefore these same 

 characters -would be more likely again to vary than the generic 

 chaiacters Avhich have been inherited Avithout change for an 

 cjiormous period. It is inexplicable on the theory of creation 

 ivhy a part developed in a very unusual manner in any one 

 species of a genus, and therefore, as we may naturally infer, 

 of great importance to that species, should be eminently liable 

 to variation ; but, on our view, this part has undergone, since 

 the several species branched off from a common progenitor, an 

 luuisual amount of variability and modification, and therefore 

 we might expect the part generally ^o be still variable. But 

 a part may be developed in the most unusual manner, like the 

 wing of a bat, and yet not be more variable than any othci 

 structure, if the part be common to many subordinate forms, 

 that is, if it has been inherited for a very long period ; for in 

 this case it will have been rendered constant by long-continued 

 natural selection. 



Glancing at instincts, marvellous as some are, they offer no 

 greater difliculty than does corporeal structure on the theory of 

 the natural selection of successive, slight, but profitable modi- 

 iications. We can thus imderstand why Nature moves by 

 graduated steps in endowing different animals of the same 

 class Avith their several instincts, I have attempted to show 

 how nmch light the principle of gradation throws on the ad- 

 mirable architectural powers of the hive-bee. Habit no doubt 

 sometimes comes into play in modifying instincts ; but it cer- 

 tainly is not indispensable, as we see in the case of neuter in- 

 sects, which leave no progeny to inherit the effects of long- 

 continued habit. On the view of all the species of the same 

 genus having descended from a common parent, and having in- 

 herited nuich in common, we can understand how it is that 

 allied species, when placed under widely-different conditions 

 of life, yet follow nearly the same instincts ; why the thrushes 

 of tropical and temperate South America, for instance, line 

 their nests with mud like our British species. On the -view of 

 instincts having been slowly acquired through natural selec- 

 tion, we need not marvel at some instincts being apparently 

 not ])erfect and liable to mistakes, and at many instincts caus- 

 ing other animals to suffer. 



If species be only well-marked and permanent varieties, we 

 can at once see why their crossed offspring should follow the 



