From the middle of the eighteenth century to the present day naturalists and psychologists have been labouring to distinguish instinct from intelligence. It is not hard to define well-marked examples of each, and to show that a typical instinct is _congenital_ (not the result of a process of education or self-education), _adaptive_ (conducive to the welfare of the organism), _co-ordinated by nerve-centres_ (thus excluding the superficially similar behaviour of the lowest animals and all plants), _actuating the whole organism_ (thus excluding most, if not all, reflex acts in the higher animals, as well as the wonderful adjustments effected by bone-corpuscles and other parts of organisms), and _common to all the members of a species or other group_ (thus excluding individual aptitudes).[30] In the same way it is easy to point out clear differences between a bird and a tree. But just as a definition which shall separate every animal from every plant has hitherto been sought in vain, so it has hitherto been impossible to frame a definition which while including all instincts shall admit no case of reflex action or intelligence. The most ambiguous cases of all are perhaps to be found in insects, where, as will shortly be explained, our information is ill-fitted to support precise distinctions.
Many naturalists entertain some form of what may be called the _use-and-disuse_ or _inherited-memory_ theory, supposing that the aptitudes of the offspring are influenced by the activities of the parent. Some cling to the belief that habits can be fixed and transmitted, and we must admit that the fixation and transmission of habits might explain a great deal. But all the evidence goes to prove that habits are not inherited at all, and that we must look elsewhere for the origin of instincts. Let naturalists who think differently try to account for the instincts of working bees or ants, which receive their psychical not less than their physical endowment from a long succession of ancestors, none of which worked for their living. Or let them try to explain the instances of spiders, insects, etc., which _after egg-laying_ practise instinctive arts for the defence of their brood, standing over the eggs, carrying them about, blocking the entrance of the burrow, etc. May we not say that it is impossible for the acts of a parent to influence the congenital instincts of offspring which have already lost connection with the mother? But surely a theory of instinct breaks down which fails to account for the expedients by which the worker-bee, the worker-ant, and the spider provide for the safety of the unhatched brood or for the welfare of the community.