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vi A THEORY OF BIRDS' NESTS 121their elders, without any regard to the use or applicability ofthe particular acts. So, in savages, many customs peculiar toeach tribe are handed down from father to son merely by theforce of habit, and are continued long after the purpose whichthey originally served has ceased to exist. With these and ahundred similar facts everywhere around us, we may fairlyimpute much of what we cannot understand in the details ofBird-Architecture to an analogous cause. If we do not do so,we must assume either that birds are guided in every actionby pure reason to a far greater extent than men are, or thatan infallible instinct leads them to the same result by adifferent road. The first theory has never, that I am awareof, been maintained by any author, and I have already shownthat the second, although constantly assumed, has never beenproved, and that a large body of facts is entirely opposed toit. One of my critics has, indeed, maintained that I admit" instinct"under the term " hereditary habit " but the;whole course of my argument shows that I do not do so.Hereditary habit is, indeed, the same as instinct when theterm is applied to some simple action dependent upon apeculiarity of structure which is hereditary ;as when thedescendants of tumbler pigeons tumble, and the descendantsof pouter pigeons pout. In the present case, however, Icompare it strictly to the hereditary, or more properly, persistentor imitative, habits of savages, in building theirhouses as their fathers did. Imitation is a lower facultythan invention. Children and savages imitate beforethey originate ; birds, as well as all other animals, do thesame.The preceding observations are intended to show that theexact mode of nidification of each speciesof bird is probablythe result of a variety of causes, which have been continuallyinducing changes in accordance with changed organic orphysical conditions. The most important of these causesseem to be, in the first place, the structure of the species,and, in the second, its environment or conditions of existence.Now, we know that every one of the characters or conditionsincluded under these two heads is variable. We have seenthat, on the large scale, the main features of the nest built byeach group of birds bears a relation to the organic structure