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•VOLUME 31, No. 3 SEPTEMBER 1954

FOURTH (FINAL) REPORT ON A TEST OF McDOUGALL'S

LAMARCKIAN EXPERIMENT ON THE

TRAINING OF RATS

BY THE LATE W. E. AGAR AND

F. H. DRUMMOND, O. W. TIEGS AND M. M. GUNSON

The Zoology Department, University of Melbourne

(Received n November 1953)

INTRODUCTION

This is the final report on the experiment, begun by us in 1932, and of which three

interim reports have already been published (1935, 1942, 1948). It was essentially

an examination of the well-known experiment of McDougall, purporting to have

demonstrated a Lamarckian effect in the inheritance of an induced light phobia

in rats.

Our experiment consisted in placing the rats into a tank of water from which

they emerged by the choice of one of two exits. Of these one was illuminated, the

other not; and a preference for the non-illuminated (dim) exit was induced in the

rats by electrifying the illuminated exit. With this apparently simple problem of

learning to avoid the lighted exit, the rats were daily confronted until they learnt

to solve it. The number of errors made by the rats was recorded and a sustained

diminution in the number of these errors in successive generations, measured

against a control series of generations, is the criterion for the operation of a

Lamarckian factor.

Over the thirty-two generations of McDougall's experiment, for which records

are available, there was such a progressive decline in the number of errors.

McDougall attributed this improvement in facility in learning to the inheritance of

the effects of ancestral training. At the time, this conclusion was justified to the

extent that no alternative explanation could be advanced to account for it. It was

this that led us, and Crew, to repeat the experiment. Crew (1936) found no

evidence of increased facility in learning during the eighteen generations of his

experiment.

TRAINING PROCEDURE

We will give a brief description of the methods we have used; fuller details are to

be found in our First Report. The apparatus was essentially as designed by

McDougall (1930). It consisted (Fig. 1) of a tank of water divided into three

parallel passages communicating with one another at the far curved end of the

tank. At the near end of each side-passage was a sloping wire ramp up which the

rats could scramble from the water. Behind a sheet of ground glass at the back of

each ramp was an electric lamp which shone down the passage and illuminated its

JEB . 31, 3 21

3o8 THE LATE W. E. AGAR AND OTHERS

communication with the central passage. The circuit was arranged so that one or

the other lamp could be lit alternately. Coupled with the lighting circuit was

a second circuit that electrified the ramp on the illuminated side; a current of

230 V., 1-2 mA. was used, with a duration of 3 sec.

A rat placed in the water at the near end of the central passage swam along it and

then had a choice of two escape routes. If it chose the bright ramp it escaped at the

expense of a 3 sec. electric shock. The rat had to learn to escape always by the dim

exit, irrespective of whether this was on the right side or the left. Facility in learning

was measured by the number of errors made, i.e. the number of escapes by the

bright exit, before it learnt to use the dim one always. A rat was held to have learnt

the task as soon as it made twelve consecutive correct runs.

Our routine procedure has been to wean the rats when 26 days old. To acquaint

Central passage

Fig. 1. Diagram of training tank.

them with the training apparatus, they were given, on the following day, six ' runs'

without illuminated ramps, after which normal training began. This consisted of

four runs per day for 5 days (the animals still being rather small) and thereafter

six per day until the task was learnt. A small proportion of rats, that had failed to

solve the problem by the 52nd day of training, were given 'special training'. They

were, almost without exception, rats that had developed the habit of going exclusively

to one ramp and were quite unable to solve the problem because of unawareness

of the alternative exit. 'Special training' consisted in forcing the animals,

usually against strong resistance, to take the correct pathway. Training of all rats

was continued after learning was complete, and until the time of mating, but was

limited to two runs per day.

McDougalVs Lamarckian experiment on the training of rats 309

CONTROLS

A fundamental weakness in McDougall's experiment was his failure to maintain

a control line of rats for comparison with his trained line. In our experiment we

instituted a proper control line, bred parallel with the trained line and under the

same conditions. All the rats were descendants of a single pair of Wistar origin.

The first generation obtained from this pair (which was not trained) was divided

into two groups, one of which was trained and became the ancestors of the trained

line (T). The other group was not trained and became the ancestors of the control

line (C). In each generation the required number of rats of the trained line was

trained and mated as parents of the next generation. In the control line some

litters were not trained but were kept as parents of the next generation; other

litters of this line were trained to provide controls to the same generation of the

trained line. These trained controls were, of course, not used for breeding. In this

way each generation of the trained line was tested against an approximately equal

number of controls, differing from the trained line only in the fact that their

ancestors were not trained.

In our Third Report we stated that genetic differences in colour pattern and

body size between the trained and control lines had appeared, and suggested that

mutations could have been responsible for the consistent superiority of the trained

line over the control between about generations 12-28. This raised the possibility

that further mutations, having a direct effect on the rate of learning, might occur.

If such mutations accumulated in the trained line their effect would simulate that

of Lamarckian inheritance. To meet this possibility we took the precaution of

maintaining from generation 41 onwards, two trained sublines (TA and TB) and

two control sublines (CA and CB). The offspring of generation 40 of the trained

line were divided into two groups. One became the ancestors of subline TA, the

other, the ancestors of subline TB. The controls were treated in a similar manner.

The two control sublines were thus joint controls to the two trained sublines; there

was no special relationship between TA and CA, or between TB and CB. It may

be stated at once that there was no evidence of divergence, in respect of facility in

learning, between the sublines during the ten generations, 41-50.

MATING AND MORTALITY

The minimum age at which the rats were mated was 85 days. By this time even the

rats which required ' special training' had learnt the task. In order that every rat,

whether it had learnt quickly or slowly, should have an equal chance of becoming

a parent, all the rats of a generation were mated at the same time.

The rats were mated without reference to their training scores. Except for

a short period, brother-sister matings were avoided as far as possible. Not all the

mated rats became parents of the next generation, for many of the matings proved

infertile, and others did not produce Utters till after the number of young required

had been obtained. This, of course, applied to both the trained and control

lines.

3io THE LATE W. E. AGAR AND OTHERS

Of the 4654 rats which started training forty-three died before they had learnt

the task. These forty-three rats have been excluded from our figures. Throughout

the whole of the experiment there were no injuries of any kind attributable to the

electric shock.

MEASURE OF PERFORMANCE

In previous reports we have discussed the problem of finding a satisfactory measure

of the performance of a group of rats as a whole. The use of the arithmetic mean

number of errors made by the rats is unsatisfactory owing to the extreme skewness

of the distribution (Second Report, Table 1) and, in any case, is invalidated by our

practice of giving 'special training' to the very slow learners. In this report, and

for the analyses of the results of the experiment, we have used the measure adopted

in our Third Report: ' The scores of the first thousand control rats were arranged

in order of magnitude, and the whole group divided into ten classes, each containing

as nearly as possible an equal number of rats, having regard to the fact that the

number of errors are necessarily whole numbers.'

The resulting distribution is shown in Table 1.

Table 1. The first 1000 controls classified according to the number of errors made

Class

Thus all the rats with training scores 0-5 errors inclusive are placed in class 1,

and so on. The arithmetic mean of the classes so obtained will be referred to as the

mean class of the group of rats concerned.

BODY SIZE IN THE TRAINED AND CONTROL LINES

In our Third Report we discussed a genetic difference in body size between the

trained line and the control. Weighings made in generations 25-28, and also in

generations 34-36, showed that the rats of the trained line were substantially

heavier than the controls. At 26 days old the difference in mean weights was

approximately 13 g. (Table 2).

Further series of weighings at 26 days old were made in generations 49 and 50.

These showed (Table 2) that there was no difference in weight between the two

trained sublines, but that the mean weights, by comparison with those of generations

previously weighed, had fallen by about 9 g. The mean weights were: females

(85) 436 g., males (70) 45-2 g. Only 6% of the rats in these two generations

weighed more than 50 g.; in generations 25-28 and 34-36, 60 % exceeded this

weight.

In the controls, the mean weights of the two sublines in generation 49 were much

the same. They conformed to those of earlier generations of controls and this was

also true of subline CA of generation 50. However, in subline CB of this generation

McDougalVs Lamarckian experiment on the training of rats 311

the mean weights were: females (23) 43-2 g., males (23) 43-8 g. These are practically

the same as those of the trained sublines.

Greenman & Duhring (1931) have recorded the weights of a large number of

rats from the Wistar Institute colony. Over a period of 4 years, eight groups of

males and females were weighed. At 25 days old, the mean weights of the groups

varied from 343 to 48-6 g. If 2-5 g. is added to the figures of Greenman & Duhring

to allow for the fact that they refer to rats 1 day younger than ours, the total mean

weights become: females (423) 43-2 g., males (455) 43-8 g.

Thus, at the end of our experiment there was no evidence of the genetic difference

in body size which previously had distinguished the rats of the trained line from the

controls and also from Wistar Institute stocks.

Table 2. Mean weight in grams, with standard errors, of rats at 26 days.

The figures in brackets are the number of rats weighed

Generation

GENERAL RESULTS OF EXPERIMENT

Data covering generations 1-36 are given in our earlier reports. Tables 3 and 4 of

the present report give the data for generations 37-50 which conclude the experiment.

The results are summarized in two graphs (Figs. 2, 3). Fig. 2 shows the

annual performances over the 20 years of the experiment; Fig. 3 gives the performances

of successive generations. In the latter figure, we have arranged the

generations as nearly as possible in groups of 4 (see Table 4), in order to minimize

chance fluctuations.*

The general result is that periods of progressively decreasing scores have

alternated with periods of progressively increasing scores, and in this the controls

have participated. Thus there was a fairly regular decrease in the number of errors

during the first sixteen generations, a slight increase in the following four and then

a further decrease until the twenty-eighth. Over the next eight generations there

was a marked increase in the number of errors, and high scores were maintained

until the 40th generation after which a further decrease occurred. Thus in spite

of the great improvement during the first half of the experiment the scores of the

• The last group necessarily contains only two generations (49 and 50) and in the first group of

controls only generations 2-4 are, of course, included.

Table 3. The number of errors made {shocks received) by each rat, the median

number of errors, and the mean class, in each of generations 37-50

(T, trained line; C, control line. S indicates that the rat qualified for special training. In the trained line the rats which became parents of the next generation are in heavy type.)

Genera- tions

No. of

rats Median Mean class No. of errors made by each rat

THE LATE W. E. AGAR AND OTHERS

last six generations were of the same order as those of generations 13-16. Throughout

the whole experiment the parallelism between the performances of the trained

and control lines was remarkable; over the last few generations the controls were

generally even superior to the trained line.

Table 4. Summary of the results of the fifty generations in

groups of four generations

Genera tions

1933 '34 '35 '36 f37 '38 '39 '40 '41 '42 '43 '44 '45 '46 '47 '48 '49 '50 '51

Fig. 2. Continuous line, line T; broken line, line C. The mean classes are those of all rats which

began training in the year referred to. The first point, 1933, includes some rats which began

training late in 1932; the last point, 1951, includes some rats which began training early in 1952.

It is unfortunate that McDougall did not publish full details of the performances

of his rats; his reports give only the arithmetic mean of the scores in each generation

and the scores of the best and worst rats. But although an accurate comparison of

the rate, and extent, of changes in learning in the two experiments cannot be made,

it is clear that the improvement which characterized our first twenty-eight generations

closely parallels that of McDougall's thirty-two generations and it seems

probable that the same factor, or factors, operated in the two experiments. But

McDougaWs Lamarckian experiment on the training of rats 315

McDougall's claim that the improvement was due to Lamarckian inheritance is

plainly invalidated first, by the performance of our control line and secondly, by

the fact that, in our experiment, the improvement was not maintained in later

generations.

DISCUSSION OF RESULTS

What is the explanation of the observed changes in the rate of learning? Selection

can be ruled out. McDougall found that improvement was continued when he

deliberately practised adverse selection and, in our experiment, it would be difficult

to explain on any selection hypothesis, why, with a standardized system of training

and mating, the direction of change in rate of learning should be periodically

reversed. The parallel performance of the trained and control lines suggests that

J I

1-4 5-8 9-12 13-16 17-20 21-24 25-28 29-32 33-36 37-40 41-44 45-48 49-50

Generations

Fig. 3. This figure shows Table 4, column Mean Class, in the form of a graph. Continuous line,

line T; broken line, line C.

the changes were related to factors, not necessarily having any genetic basis, which

influenced the rate of learning.

In our First Report we listed six such factors : (1) the severity of the punishment,

(2) vigour, (3) intelligence (ability to learn by experience), (4) the strength of the

right or left habit, (5) 'venturesomeness', (6) chance factors not causally related

to the learning process at all. Subsequent analysis of the data has shown that yet

another factor needs consideration and that the performance of the rats was influenced

by the season of the year in which they were trained. The separate or

additive effects of these seven factors could explain the great variation in the performances

of individual rats and the differences between particular generations,

but only the first two, i.e. severity of punishment and vigour, seem to offer any

basis for an explanation of trends of improvement or decline extending over a

number of successive generations.

(a) Seasonal effect

When the rats of the whole fifty generations of the two lines are grouped according

to the month in which they commenced their training and the mean class

is calculated for each of the 12 months, it is found that, starting in February, the

316 THE LATE W. E. AGAR AND OTHERS

means increase regularly to reach a maximum value in July and then decrease

regularly to a minimum value in November (Fig. 4, Table 5).

In order to find whether this seasonal factor had operated throughout the

experiment, similar analyses were made after dividing the experiment into four

5-year periods. In the early months of the year the results were erratic, but from

March-April onwards generally conformed to the pattern of the previous analysis

and showed consistently that rats which commenced their training during the

winter months of June, July and August were at a disadvantage by comparison with

those which commenced training in November and December.*

This effect may have been due, in part, to our failure to maintain a constant

temperature in the colony room and in the water in the tank. The only precautions

taken were, that in winter, the room was heated and warmed water was used in

the tank. In neither case, however, was the temperature raised to summer levels.

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Fig. 4. The mean classes are those of the rats which began training in the month and period referred

to. , 1932-52; , :932-7J • 1938-42; , 1943-7;--, 1948-52.

The lower water temperature during the winter may have had a slightly adverse

effect on the performance of the rats, for Wever (1932) and Hack (1933) have found

that the rat's incentive to escape from immersion varies with water temperature,

being greatest at low temperatures. At lower temperatures (10-200

C.) the rats

after being placed in the water swam strongly and directly to the landing platform.

At temperatures from 30 to 400

C, Wever states that 'many of the animals did not

head for the goal immediately but spent some time in casual exploration'. Hack

describes the rats at 37-5° C. as showing an 'inquisitive attitude' with frequent

reversals and re-entries into the blind alleys of the maze. This latter type of behaviour,

we believe, facilitated learning in our experiment, but we are disinclined

to attribute seasonal variation in performance to differences in water temperature;

for the greatest range in temperature would not have exceeded 12-220

C. and the

• The main divergences from the pattern are the low values of the class mean in the May group, 1948-52, and the June group, 1932-37. In both groups the number of rats was small—considerably

less than half the average number for the other months.

Table 5. The rats of lines T and C combined classtjied accordi7lg to the month in which thq, began their training

318 THE LATE W. E. AGAR AND OTHERS

maximal temperatures would have occurred in summer, not in November, which

was the period of minimum scores. But in any case, seasonal effects cannot possibly

account for trends, lasting over a period of some years, and it is primarily with

these that we are here concerned.

As will be shown later, we do attach great importance to the effects on learning,

of differences in behaviour in the tank, but we believe that these differences were

largely the expression of variations in the health and vigour of the rats.

(b) Severity of punishment

In his preliminary experiments, McDougall found that the strength of shock had

considerable effect on the rate of learning. Thus if the severity of punishment tended

to wax and wane over long periods the performance of the rats would show corresponding

trends. We are satisfied that this effect did not operate in our experiment.

The importance of a standardized punishment was recognized from the beginning

and the precautions taken to ensure this (see First Report) have been maintained.

The electrical installations, as shown by periodical tests, have remained in perfect

order.

(c) Vigour

McDougall, Crew and ourselves have all noted that less vigorous rats tended to

learn more quickly. Crew attributed this to their receiving more severe punishment.

We, however, agree with McDougall that it was mainly a result of their slower and

more hesitant progress through the water which gave them more time to perceive

the situation.

McDougall, while admitting the relationship, was inclined to minimize its effect.

He wrote (1938, p. 374): 'very great vigour and liveliness is a little unfavourable

to quick learning and a somewhat diminished or less-than-average vigour is probably

slightly favourable'. Crew, having noted that poorly developed, feeble rats learnt

quickly, suggested that ' as a general rule, the more vigorous the rat the higher the

score may be expected to be'. In our First Report we produced evidence to support

this view. Fifteen rats of the 3rd-5th generations which, before the 10th day of

training and before learning, had been noted on the training cards as 'weak' or

' undersized' had an average score well below the general average of these generations.

There was also evidence from generation 17 which was severely debilitated

by a mite infestation. After this had been controlled and the diet changed there

was an extraordinary improvement in the health of the colony and a striking change

for the worse in training performances; forty-five rats trained prior to the eradication

of the mites had a class mean of 3 29; thirty-five rats born afterwards had

a class mean of 6-40. But in the following generations, although the rats remained

in a good condition, there was a return to low training scores and we were then

inclined to accept McDougall's conclusion that variation in health and vigour

could not explain the fluctuations in the rate of learning.

Further experience led us to revise this opinion. During the course of the

experiment, the general health and fertility of the rats varied considerably. At

irregular intervals the colony went into a decline extending over several generations

McDougalVs Lamarckian experiment on the training of rats 319

and then, for no apparent reason, regained its health and vigour. McDougall

evidently had the same experience for he refers to 'waves of decline of vigour'.

Greenman & Duhring (1931), working with a selected group of rats at the Wistar

Institute, report large weight fluctuations over a succession of generations so that

changes in the general physique of the albino rat may occur, even in colonies

maintained in the most favourable environment. The occurrence of such changes

in our colony, coupled with the facts set out above, justified a detailed analysis of

possible relationships between health and training scores.

As the need had not been foreseen, proper records of the health of the rats were

not kept and the data available are therefore few. We have, however, fairly complete

fertility records, and as fertility is correlated with health these have provided us

with an indirect measure of health. Since all the members of a single generation

were mated at the one time, they were all therefore given an equal chance of

reproducing; and it is justifiable to use, as an index of fertility, the number of

fertile rats in each generation, expressed as a percentage of the number mated.*

The rats were weaned at 26 days and commenced their training on the 28th day.

The great majority (about 80%) learnt during the first fortnight of training, i.e.

between the ages of 31 and 42 days, when their general health would be determined,

in large measure, by the nursing capacity and therefore the general health of the

mother.

Thus if rate of learning were influenced by the health of the rats one would

expect a correlation between fertility, used as a measure of health, of one generation,

and the training scores of the next.

The first analysis was made in respect of generations 1-40, i.e. for the period

prior to the splitting into sublines. The coefficient of correlation, for the trained

line was +040 and for the control line +042. Both values are significant at the

1 % level, and fertility is therefore shown to be positively correlated with high

training scores.

The analysis was then extended to include the whole fifty generations, treating

the trained rats and the controls each as a single population. For the trained line

there was again a positive correlation significant at the 1 % level. For the control

line the correlation was positive but was not significant.

When the control sublines of generations 41-50 were analysed separately, it was

found that there was no significant correlation between fertility and training scores

in either subline, but that in subline B, such correlation as occurred, was negative.

This latter fact was not wholly unexpected for until the 48th generation it had been

obvious at the time of training that subline B was atypical in that, while fertility

was fairly high, the rats were undersized and poor in health. Their inferiority was

indicated by their small litters. In every generation from 41 to 48, the average litter

• In the trained line our records show, as fertile, only those rats whose offspring were taken into training. When fertility was high, some first litters were discarded without their parentage being

recorded and the fertility index would thus give an underestimate of the true level of fertility. This situation did not often arise, and, in fact, the value of the index for the trained line was not signi- ficantly different from its value for the controls, where all first litters were required, either for breeding or training.

320 THE LATE W. E. AGAR AND OTHERS

size in subline B was smaller than in subline A. For the eight generations combined

the average for B was 7-3, for A 93.

While there are grounds for regarding control subline B as atypical, the occurrence

of a negative correlation between fertility and scores is disconcerting. It does

not, however, discredit the highly significant correlation established for the first

forty generations of the control line and for the whole fifty generations of the

trained line.

The possibility that there might be a correlation between fertility and rate of

learning was considered by both Crew and McDougall. Crew pointed out that if

there were a positive correlation between fertility and quickness, i.e. the reverse of

the one established above, it could explain the progressive improvement of

McDougall's rats. McDougall had been fully aware of the significance of the point

made by Crew, but such evidence as he collected, supports our conclusion. In his

water-maze experiments, several attempts to isolate a superior stock, by breeding

from selected quick learners, failed on each occasion because of the infertility of

the superior rats. He had the same experience in his first selection experiment with

quick learners in the tank.

We have established a positive correlation between the fertility of one generation

and the scores of the next but it is difficult to believe that there could be any direct

causal relationship between them. They must have been connected by a third

factor. The basic premise of the foregoing analysis is that this third factor was the

general health and vigour of the rats. If it be accepted that healthy vigorous rats

are more fertile than less vigorous ones, then the above correlation indicates that

the more vigorous the rat the higher its expected training score.

Tryon (1929, 1932) has suggested that the reverse relationship may hold for

maze-learning. By breeding rats selectively, according to their ability on a maze,

he developed a strain of 'brights' and a strain of 'dulls'. Over a number of

generations he found that both lines showed a progressive improvement in learning

ability. Tryon was inclined to attribute this improvement to increased vigour of

the rats. Were this so, it would not necessarily conflict with our conclusion on the

influence of vigour on learning in the tank. Krechevsky (1932) has analysed the

performances of Tryon's two strains, and has concluded that the difference between

them was related specifically to maze learning. The 'brights' learned quickly

because they used 'spatial hypotheses'. The 'dulls' used 'visual hypotheses'.

Drew (1939) has pointed out that this, while penalizing them on a maze, would have

favoured them in a Ught-o^scrirriination test, and that Tryon's 'dulls' would

probably have been quick learners in the tank. The two learning situations were so

different that vigour could have been favourable in one, and a handicap in the other.

The training scores of individual rats were necessarily influenced by a variety of

factors and all of these, no doubt, played some part in causing the changes in the

average rate of learning which occurred during the course of the experiment.

We believe, however, that the major changes, that is the changes involving a progressive

improvement or decline extending over several generations, were due

primarily to changes in the general level of health in the rat colony. The cause of

McDougalVs Lamarckian experiment on the training of rats 321

these fluctuations in health of the colony is quite unknown. It may be that they

result from infection, and that recovery from a decline involves a selection process,

in which the enfeebled strain is eliminated through diminished fertility.

In retrospect it seems that we have been less successful than Crew in standardizing

the factors that cause variation in the rate of learning. This has, however, had the

positive advantage that it has enabled us to obtain the effect of a progressive improvement

in learning rate that McDougall found. McDougall attributed it to the

operation of Lamarckian inheritance. Our own results forbid this interpretation

for the effect is not sustained, and is displayed in equal measure by the controls.

SUMMARY

This is the final report of an experiment of 20 years' duration, in which we have

repeated, in its essentials, the well-known experiment of William McDougall purporting

to reveal a Lamarckian inheritance of the effects of training on rats. The

test is one involving light discrimination, and McDougall recorded a steady improvement

in the rate of learning on a succession of 32 generations; but he omitted

to check the results against a properly conducted control.

Our experiment confirms McDougall to the extent that we too have obtained

long duration trends of improvement in learning-rate (Figs. 2, 3); but we find that

the effect is not sustained, and that it is, moreover, shown also by a control experiment,

using animals of untrained ancestry. This forbids a Lamarckian interpretation.

Statistical analysis of the data indicates that the ' condition' of the rat markedly

affects its speed of learning, and that progressive changes in learning-rate, over

a succession of generations, are in reality correlated with the health of the laboratory

colony, which is subject to periods of decline and recovery.

REFERENCES

AGAR, W. E., DRUMMOND, F. H. & TIEGS, O. W. (1935). A first report on a test of McDougall's Lamarckian experiment on the training of rats. J. Exp. Biol. ia, 191-211. AGAR, W. E., DRUMMOND, F. H. & TIEGS, O. W. (1942). Second report on a test of McDougall's Lamarckian experiment on the training of rats. J. Exp. Biol. 19, 158-67. AGAR, W. E., DRUMMOND, F. H. & TIEGS, O. W. (1948). Third report on a test of McDougall's

Lamarckian experiment on the training of rats. J. Exp. Biol. 35, 103-22.

CREW, F. A. E. (1936). A repetition of McDougall's Lamarckian experiment. J. Genet. 33, 61-102. DREW, G. C. (1939). McDougall's experiment on the inheritance of acquired habits. Nature, Lond.,

143, 188-91. GREENMAN, M. J. & DUHRING, F. L. (1931). Breeding and Care of the Albino Rat for Research

Purposes, 2nd ed. Philadelphia, U.S.A.: The Wistar Institute of Anatomy and Biology. HACK, E. R. (1933). Learning as a function of water temperature. J. Exp. Psychol. 16, 442-5. KRECHEVSKY, I. (1932). Hereditary nature of hypotheses. J. Comp. Psychol. 16, 99-116. MCDOUGALL, W. (1927). An experiment for the testing of the hypothesis of Lamarck. Brit. J.

Psychol. 17, 267-304.

MCDOUGALL, W. (1930). Second report on a Lamarckian experiment. Brit. J. Psychol. 20, 201-18. MCDOUCALL, W. (1938). Fourth report on a Lamarckian experiment. Brit. J. Psychol. 28, 321-45;

RHINE, J. B. & MCDOUGALL, W. (1933). Third report on a Lamarckian experiment. Brit. J. Psychol.

34, 213-35-

TRYON, R. C. (1929). The genetics of learning ability in rats. Univ. Calif. Publ. Psychol. 4, 71-89. TRYON, R. C. (1932). The inheritance of maze ability. Rep. Afith Annual Meeting A.P.A.

WEVER, E. G. (1932). Water temperature as an incentive to swimming activity in the rat. J. Comp.

Psychol. 14, 219-24.