Glancing at instincts, marvellous as some are, they offer no greater difficulty than do corporeal structures on the theory of the natural selection of successive, slight, but profitable modifications.

We can thus understand why nature moves by graduated steps in endowing different animals of the same class with their several instincts.

I have attempted to show how much light the principle of gradation throws on the admirable architectural powers of the hive-bee.

Habit no doubt often comes into play in modifying instincts; but it certainly is not indispensable, as we see in the case of neuter insects, which leave no progeny to inherit the effects of long-continued habit.