A. Guinea pig. — The guinea pig at birth is well covered with hair; its eyes are open; smell, touch, and taste ap- parently all function; movement is not well coordinated and muscular weakness is apparent. It is able to run at birth, but after running a short distance and stopping, one leg is left sprawling behind it. There is no fear at birth of SERIAL UNFOLDING OF INSTINCTS 129 an approaching object, such as the hand held in front of the eyes, nor of persons. A shrill squeak, like that of a rat, caused first an instantaneous jump with a twitching of the muscles. This is the immediate reaction to such a stimulus. It may be followed by hiding under the mother: A carrot or other vegetable food produces no positive re- sponse at birth, but after the end of the first day these objects will be eaten. Small guinea pigs put outside of the experimental cage in w^hich the mother was placed ran around quite freely and contentedly for an hour or more without the mother. In one case where the young one was removed from the mother for five hours, it gnawed a little at the wire. The mother does not seem to be a specific stimulus to the young, but after they have been returned from an absence of several hours they will seek her and suck. At 38 hours, the guinea pig's squeal, in its infantile form, is fully developed. The movements are almost as well coordinated as in the adults; there is great activity. The movements around the cage are similar to those of the adults when hunting food: the fore-feet creep forward, the belly flattens to the ground, and the hind part of the body is dragged forward. The peculiar movements of the guinea pig, so characteristic in the first three weeks of existence, begin to appear on the second day. At the age of 62 hours evidence of the mother acting as a specific stimulus is given in the act of the young in making real attempts to get to her through the wires of the cage. The small guinea pigs rarely play together. It is probable that the guinea pig at 3 days of age is as mature as the white rat at 23 to 27 days of age.
B. Rat. — In marked contrast to the precocious develop- ment of the guinea pig stands that of the white rat.
First day. — The young white rat is born destitute of hair. There is no external ear and there is a membrane over the eyes. The nose seems to be fairly well developed. Their movements, except sucking, are incoordinated, the mother crouching over them. When held in the hand they roll up into a ball; when placed on their backs outside of the nest they hitch and kick and wriggle over upon the 130 STUDIES UPON INSTINCT belly or the side. They are generally unable to maintain the belly position for any length of time. When lying upon the belly they stretch out their paws in a turtle-like man- ner. The tail is often moved. Vocalization consists in a clucking sound, a fine high-pitched squeak, and a sort of chirp. Avoiding types of reaction are present such as brushing and pushing away with the paws; averting the head; movement of the whole body, etc.
It will thus be seen that there is very little that is defi- nite in the way of instinctive activity. From birth until the fifth day there seems to be a steady increase in motor development. But no new instincts appear.
Fifth day. — The rats crawl with considerable vigor on the fifth day.
Twelfth day. — Walk, but unsteadily. Characteristic act of " face washing " appeared on this day in fairly perfect form.
Fourteenth day. — Running, climbing, characteristic movement of rearing on hind legs, lifting head, and sniff- ing. It is at this age that the young becomes sensitive to sounds.
Fifteenth day. — The eyes open. When removed from nest, can run back to mother with great rapidity. Scratch- ing in sawdust to get back to mother when entrance was closed with sawdust was clearly noted.
From this period until full sexual maturity is reached, about the sixty -fifth day, the development of play activity is a most interesting phenomenon to watch. They are ex- ceedingly active during this period. Small, from whose records the above notes are in part taken, states that the whole repertoire of play activity is complete at the age of 25 days. He enumerates the play acts as follows: running, jumping, climbing, fierce sham fights with biting, clawing, running over the mother and biting her ears, dig- ging in corners, gnawing at the cage, sex movements (mounting one another — not complete). This catalog of play is very incomplete, as Small himself admits.
Of the instincts in this animal not exhibited until after sexual maturity is reached — such as construction of the SERIAL UNFOLDING OF INSTINCTS 131 nest, caring for the young, assembling them, storing food, etc. — very little is known.
C. The monkey. — One is bewildered by the very com- plexity of the responses in the young monkey, just as is the case in similar observations upon the child. There seems to be great progress in the development rather than the putting on of very definite instincts at any given time.
First week. — On the first day the little monkey was very still, moving only when forced to do so by the movements of the adult. However, certain reflexes seemed to be well established even at this age. Those observed were sucking, grasping, nuzzling, crying, sneezing, winking (not in re- sponse to visual stimuli), incoordinated movements of the legs when the stimulus for grasping was removed; and an increased muscular tension of the legs in response to the mother's movements. During the second day there ap- peared to be a slight unadaptive response to sound. (In the human infant this varies in the time of appearance from the second to the tenth day.) By the third day his strength had increased so that his grasp upon his mother's hair had become secure. The head and eyes were at this age first turned to follow a moving object. Two days later appeared the reflex grasping at a visual object.
Second iveek. — By the end of the first week his muscular activity was greatly increased, although most of his move- ments were still of the non-adaptive type. Complex co- ordinated movements of the limbs, as in jumping, appeared, as also those connected with reaching out towards moving objects. The scratching reflex was established. On the eleventh day he reached out and grasped small objects. By this time his response to sound indicated some degree of localization. On the twelfth day he first attempted to walk. The movements were very badly executed and showed a lack of coordination.
Third week. — The third week, the first week of walking, was marked by a tremendous increase in muscular control. The movements of the larger muscles of the limbs became fairly well adjusted to the demands made upon them. The 132 STUDIES UPON INSTINCT first definite play activities appeared with contraction of the facial muscles in pursing the lips.
Fourth iveek. — Play rapidly changed from simple pull-i ing and grasping to the more complicated stalking and cap- turing of objects in the cage. This was closely connected with his increased interest in scraps of solid food, which he first attempted to eat at this time. The scratching re- flex was perfected. The very complex behavior displayed by the adult in this act appeared quite suddenly in the young animal. There is some evidence to show that habits of reacting to food by sight arose within a few days after he first tasted it.
Fifth week. — Apposition of the thumb and fingers was first observed, muscular coordination spreading periph- erally. He attempted to draw the mother into his play.
Sixth iveek. — Movements connected with attack and de- fense appeared, as shown by his violent attempts to shake and tear apart the cage. This is a very characteristic act of adults. The thumb was used much more in grasping.
Seventh week. — Movements connected with care of the body, removal of insects, etc., were first observed.
Eighth iveek. — The use of the cheek pouches was ac- quired.
Ninth week. — Different vocal sounds were first distin- guished. The method by which they were learned is not known, but from their sudden appearance they seem to be instinctive rather than imitative. Stroking of the sex organs resulting in erection was noted during this week.
Fifteenth iveek. — From the tenth to the fifteenth week the progress noted was almost entirely motor. Play activi- ties were still very marked but unadaptive movements per- sisted.
Approximately one year. — The group connected with the act of reproduction appeared very suddenly at the age of oiie year. So far as could be seen all of the essential re- flexes were perfect.
D. Sooty tern. — The young sooty is born in a very helpless state. On the 7th of June eight young sooties were taken, all born on that day. They differ slightly as SERIAL UNFOLDING OF INSTINCTS 133 regards their development. This is due to the fact that the birds dwell for varying lengths of time in the shell after it has become pipped. In passing through the bushes when the birds are beginning to hatch one often hears quite lusty '' peeping," and on looking about to find the young bird, discovers that the noise comes from birds in the shell. Whether or not they are fed at this age is not known. Sometimes they live as long as two days in the shell with only the beak protruding. At this stage in their develop- ment they make no response to the warning cry of the adults as they do later on. They go on '' peeping " lustily after the adults have flown. The peculiar protective atti- tude of the young birds has been mentioned already, that is, the ability to lie outstretched and perfectly motionless. If left for a time in this attitude, they begin to " peep " as soon as the sun's rays become oppressive. This means of protection persists until locomotion makes possible a more effective method. When the protective attitude disappears and locomotion becomes possible, the birds run to cover when disturbed. Hiding is never in any sense complete; indeed, the reaction seems to be almost thygmotactic. The moment the young bird can put its head in the crotch of a limb or get its body in contact with some solid object, locomotion ceases.
First day. — The young birds which were captured showed apparent signs of fear. Moving the hand quite rapidly near them, as in offering a piece of fish, caused them to dodge quite noticeably. The instinctive cry is a lusty " peep-peep." They are w^ell developed but clumsy. The wings droop and the birds have difficulty in standing; co- ordinated sitting positions are likewise almost impossible. The birds are somewhat hard to feed during the first day, but after this age the process is easier wdth them than is the case with the noddies of a corresponding age. In the case of the sooties, as was not the case with the noddies, the mere sight of the food will cause the bird to open its beak. Light contact will likewise cause the beak to open. Once the food gets into the mouth the rhythmical move- 134 STUDIES UPON INSTINCT ments of swallowing follow perfectly. Some locomotion is possible. They take a few wobbly steps with wings down and legs wide apart. Fairly well coordinated swimming movements are present, but the heavy down with which they are covered soon causes the birds to become waterlogged and they sink. As the salt water begins to enter the mouth the bird raises its beak higher and higher and shakes out the water vigorously, crying lustily the while. One of the young birds taken on that day had not emerged from its shell; only its beak and the base of the beak protruded (the eyes being covered by a membrane). Almost any call from the observer would cause this bird to " peep." Small minnows were offered and swallowed. The contact of the minnows caused the beak to open. The shell was removed; the bird made adaptive movements as if completing the process itself. After removal the bird was very insecure in its movements.
Second day. — The difference in growth is remarkable. Birds can waddle around rapidly and maintain upright position fairly well. They follow moving objects with the head and eyes quite easily. Will dodge very quickly if hand is suddenly extended towards them. Some tendency present to nestle under one another, under hands and under folds of the cloth. They are beginning to peck at one another, the bare wings being the spot usually at- tacked. Feeding is quite easy. Moving the finger, fish, or bits of straw rapidly across the beak will cause the beak to open wide. "Whole minnows are swallowed with ease. The birds sleep a great deal of the time. The adult birds flying over the experimental cage and crying are invariably answered by a '' peep " from the young ones. Even on the second day feeding is an active process with them. A fish dangled in front of them will cause them to strike at it. If the beak fastens upon the fish in the center of the body the young bird, by a peculiar twist of the head, will suddenly right the fish and swallow it head first. It is safe to say that at the end of the second day the young sooty is sufficiently developed to take any fish from the beak of the parent which is likely to be found there. Even SERIAL UNFOLDING OF INSTINCTS 135 at this early age the birds are found peeking at one an- other's mouths or at a piece of food which another is swallowing. Pecking at one another in a way forceful enough to suggest the beginning of the enormously impor- tant fighting instinct was, however, not noticed up to this time. Any signs of fear which might have been present the first day have entirely disappeared by the end of the second.
Third day. — The birds have begun to run towards the observer when he approaches them and calls. They will lustily answer his " peep " at any hour of the day or night. The fighting instinct appeared today in almost complete form. Two of the young birds faced each other and began striking simultaneously. A firm hold is taken on the body and maintained, the victor all the time shak- ing his opponent as does the adult. Not the slightest sign of play has yet manifested itself. The birds as yet show no discrimination as regards what objects they peck at: they are as likely to attempt to swallow the finger as the fish. Very minute objects, however, such as specks of dirt and hairs, are not noticed by them.
Fourth day. — The birds are healthier and larger than those in the field. Three birds, 4 days of age, were taken from the field to compare with the birds reared in captivity. These birds were exceedingly wild. They were kept in captivity for 2 or 3 weeks, but while showing im- provement in this respect, they never wholly lost their fear of the observer. The sleeping and lounging attitudes of the first lot captured are very striking. In most cases, after a full meal is taken, they go to sleep. When lying down one leg (at times both legs) is stuck out so as to give the ventral surface of the body contact with the ground; head is completely outstretched and laid on one side; eyes are closed. At other times the bird rests its rump on the ground, using the legs as a prop, and dozes in this position. As it dozes its head falls to the right or to the left or vertically downward between the legs. When the beak strikes the ground the eyes are partially opened, the head is raised again, and the process is repeated.
136 STUDIES UPON INSTINCT Seventh day. — The bird's feeding movements are won- derfully fast. The extremely rapid movements of the (live) fish are not more rapid than the movements of their beaks. There is little improvement in discrimination to be noticed. Today, while feeding some of the birds, a tug w^as felt by the observer from behind, and on turning around it was found that one of the birds had swallowed about 3 inches of his handkerchief and was straining every muscle to force the rest down! The young birds were offered lemon peel today: it was snapped up by all of them greedily. They continued to snap and swallow the lemon rind for 3 to 4 trials, and refused thereafter to open the beak for it. Fighting is furious and prolonged. The young birds are exact replicas of the old in this respect. All that is necessary to start a fight is for one bird to come within striking distance of another.
Eighth day. — The young birds began to dig holes in the sand, using exactly the same movements which are em- ployed by the adults in digging the nest, except that the young birds do not shape the hole with the breast as the adults do the nest. By digging such a hole the bird se- cures a surface which is damp and cool. The holes are usually dug near some solid object, whether because of greater coolness there (temperature reaction), or through some thygmotactic tendency (tactual reaction) we are not able to state. A new instinctive reaction was observed today which is continued from this time on. A bird stand- ing still will suddenly hop an inch or two in the air and come down in the same spot (first step in flying?). De- tailed records of the development of these birds were kept until they were 30 days of age, but on account of the great similarity of these later records to the above, we shall not cite them in detail. Experiments on learning to respond adaptively to the common objects in the environment were made from day to day, but all such tests gave results very similar to those which have already been described. After the birds had learned to pick up fish from a dish, it was possible to put seaweed, grass, bits of coral, etc., in with the fish. At first these were taken eagerly, but SERIAL UNFOLDING OF INSTINCTS 137 after a few trials the birds learned to take the fish and leave the debris. Such habits, however, never became very accurate. When very hungry the birds would attack objects other than the fish. It may be found of interest in this connection to say a few words about the way the young birds in captivity and those in the field spend the greater part of their time. We have already remarked upon the fact that there are no signs of play. This was as nearly true of the birds in the field as of those observed in captivity. The only reaction which it is at all possible to consider a playful one is the one already mentioned, viz., the frequent hopping up and flapping of the wings. It is a question whether Gross' theory of play after all fits the facts in any genus of animals except that of mammals. Certainly the facts which we could gather both with ref- erence to the noddy and sooty terns do not lend support to this theory. These birds certainly lead an instinctively complex life. Surely the picking up of a live fish darting over the surface of the water is as complex an act as the catching of a mouse. And yet it is impossible to practise this act in play. Probably the instinct is perfectly and completely called out the first time the birds see a minnow darting over the surface of the water. The birds spend the greater part of their time in doing absolutely nothing. They will lie outstretched in their sand holes, getting up at times to stand stock still for an hour or more, or to doze with head bent down. At intervals they peck at their feet and occasionally preen their feathers. In the field the routine is broken by the call from the parents which have returned to feed the young. At other times the young birds get hungry, ^t such times they begin to cry lustily and go up to the parent (which is likewise standing or fighting in the neighborhood) and beg for food by '' peep- ing " and by striking at the parent's beak. Getting noth- ing, the young birds will wander off for 10 or 15 feet and lie down again in the shade. Frequently they engage in fighting with other young birds. Occasionally they will attack adult noddies which drop down to gather sticks. The young sooty never attacks an adult sooty. As the birds 138 STUDIES UPON INSTINCT get older they begin to fly short distances. At the end of 35 days they can cover 30 to 40 feet very rapidly by a series of flying hops.
III. Quantitative study of improvement of instinc- tive function. — Under I we considered the initial per- formance of several instinctive acts. Much has been said and written about the degree of perfectness of such acts and about the rapidity of their rate of improvement when exercised. "Within recent years efforts have been made to obtain a quantitative report on the initial accuracy of such acts and their rate of improvement.
Breed has presented an interesting investigation of the pecking reaction of the chick. For purposes of study he divides the act into (1) missing, (2) striking, (3) seizing, and (4) swallowing. The curves showing the rate of im- provement in swallowing (4) are the most instructive and interesting in this connection, since under the term swal- lowing he includes the whole reaction, i.e., successful strik- ing, seizing, and swallowing in any errorless chain of movements. On the second day the chick is presented with small grains at which it begins to peck. The number of correct reactions the chick makes in 50 attempts to take up the grains is noted. The same routine is repeated each day for 25 days. Thus it was found that the average number of correct (complete) responses obtainable on the second day in one group was 10.3; on the third day, 28.3; on the seventh day, 38.3; on the sixteenth day, 43.2. Ini- tial accuracy of the whole act is thus low, but the increase during the first few days of practice is enormous. Fig. 29, curve IV-E, portrays the result Breed obtained from a group of 6 chicks. (In the same figure, curve IV-F, is shown a similar set of records of chicks, which had been allowed to watch the pecking of older birds. This curve will be discussed on p. 141.)
Granting that we can measure the degree of accuracy and general perfectness of an instinctive act, the question of interpretation still arises. Lacking experimental evi- dence, it is easily possible for one to affirm that the in- crease of the perfectness of the act comes not by virtue of QUANTITATIVE STUDY OF INSTINCT 139 the exercise or practice of the function, but rather by maturation of the sensory motor arcs (including here de- velopmental or growth processes in receptor, system of conductors, and the effector). The obvious way to control these factors is to prevent those arcs from functioning until maturation in them has partially advanced. We can so / ^ ^ L- -^ N ^ 40 1 X / ^^ 35 V:E.
// ; "^ / // IS // \ s s^ y .^l■r _^ < "^ XT"-' >- -— — Cays i Fig. 29. A Compakison of Cueves of Development of the Pecking Instinct to Show the Possible Effect of Social Influence Distances along axis of abscissse represent days of age; distances along axis of ordinates, the number of a given type of reaction in a daily series of fifty pecking reactions. Curves III-E and III-F show respective!}^ the rate of decrease in the number of reaction 3 for groups E and F; curves IV-E and IV-F show the improvement in accuracy of reaction 4 for the same groups. (After Breed, Behavior Monograph, Ser. No. 1, p. 31.)
then test the accuracy of the function under the two condi- tions of practice and non-practice. The comparison of the two sets of records ought to enable us to separate the increment due to practice from that due to maturation. Some experiments have been made with this object in view: Fig. 30 shows the development of the normal pecking reaction in chicks and the development where pecking has been artificially prevented for 3, 4, and 5 days previous to the first tests. The curves were obtained in a way similar to the ones just discussed. The number of perfect, complete reactions were recorded in each daily set of 50 trials. It will be seen that the standard curve (S), beginning on the second day, shows that the average STUDIES UPON INSTINCT number of correct responses on that day was 8.7; on the seventh day, 36.7; on the twentieth day, 40.8. This stand- ard curve is based on the average of 21 chicks. Curve I shows what happens when pecking is prevented for 3 days; curve III likewise for 3 days; curve IV, for 4 days; and curve V, for 5 days. During the days in which pecking DAYS T Fig. 30. Curves Showing the Course of Development of the Pecking Instinct After Artificial Delay S, standard curve, representing rate of improvement in accuracy under natural conditions. I, III, IV, and V, curves for corresponding groups of chicks in which the action of the instinct has been artifi- cially prevented for three, three, four, and five days, respectively, previous to the first tests. (After Breed and Shepard, Jour. Animal was prevented, the animals were watered and fed artificially and confined to the dark room. In general it may be seen from these curves that the initial efficiency is low in every case, no matter what length of time is allowed for matura- tion. The number of normal responses on the first day (i.e., when the chicks were 2 days of age) was 8.7 out of a possible 50. The average number of perfect responses in the group in which pecking was prevented for 1 day was 5.5; for 2 days, 3.1; for 4 days, 8.7; and for 5 days, 2. No one could have predicted that a 5-day-old chick, prevented from previous pecking, would start in with an accuracy no EFFECT OF SOCIAL INFLUENCE 141 greater than that found in chicks 24 hours of age. The next interesting point in the tests on the chicks which were prevented from pecking appears when it is seen that the curves showing their rate of improvement very quickly ad- vance to the level of the standard (S), only 2 days of practice as a rule being necessary to effect this.* From these experiments it seems safe to conclude that the enor- mous increase in the accuracy of pecking during the first three or four days, which is always observable in chicks which are brought up under normal conditions, is due to the effect of practice as we ordinarily understand the term. IV. Modification of instinct through social influ- ences.— Several interesting experiments have been carried out on young birds to show the types of song and call notes which they exhibit when reared in isolation from adults of their own species, and to show the effects of the song and call notes of a given species upon the young of another species. It is admitted by all that the various birds, such as the robin, the bobolink, and the oriole, when reared