This series of three papers all published in Proceedings of the Royal Society and communicated by MacBride established that food preference to match the prevailing source could be passed from parent to offspring. The background to the work is best left to Dorothy Sladden’s own words in the introduction to the first paper.
The question of the possibility of the transference of an acquired habit from parent to offspring is of fundamental biological importance. That a series of experiments of this nature could be devised to produce a.conclusive answer to this question was originally suggested to me by Professor E. W. MacBride.
In 1912 Professor MacBride working in conjunction with Miss Jackson (1915) began an investigation into the inheritance of colour in the stick-insect Carausius (Dixippus) morosus. During this investigation, which extended over a period of three years, as Professor MacBride has informed me, great difficulty was experienced in obtaining adequate supplies of privet (the food on which the insects were being reared) during the winter months. On account of this the insects were induced, not without difficulty and practically by a process of starvation, to feed on ivy. In this way a number of insects were lost, a few, however, reached maturity and eggs were deposited. When the young emerged from these eggs they took readily to the ivy. This seemed to indicate the inheritance of an induced habit, and it was felt that it would be of interest to conduct further investigations on these lines. Four years later Ling Roth (1916) who made a study of the growth and habits of the stick-insect, experienced the same difficulty in procuring privet during the winter and rediscovered the fact that under these circumstances ivy could be substituted.
In attempting to induce a new food-habit it is necessary to employ an insect which is not rigidly monophagous and the stick-insect complies with this requirement in that it is polyphagous as shown by both MacBride and Roth. The usual food-plant employed in this country is privet.
The next difficulty encountered in any such experiments is the effect of inter-breeding which may introduce concealed characters from one or other parent into the offspring. The stick-insect is for long periods parthenogenetic, the males which are smaller than the females appear only at intervals. so that no crossing can occur during the duration of the experiment. The young when hatched from the egg closely resembles the adult except for size and colour. As in the lower Insecta there is no complete metamorphoses such as leads to the intervention of pupal stages, and the changes conditional on the growth of wings, which occur in Insecta generally, are absent, since the adult Carausius is wingless.
In a problem of this nature it is necessary to employ large numbers of offspring and to extend the work over a number of generations. The life-cycle should consequently be reasonably short, Carausius morosus having a life-cycle of 9 to 10 months and producing an average of 513 eggs per parent individual (Roth, 1916) fulfils these requirements.
The stock of stick-insects she used had been kept in Britain for at least 20 years and had been fed exclusively on privet. Two types of experiment were done. The first, ‘presentation tests’, involved offering the novel food, ivy, over a period of 1-3 days after the insect was hatched. If the ivy was untouched, it was withdrawn, privet given for 1-2 days and the test with ivy repeated. This sequence was repeated until the insect ate ivy. For the first paper alone, she tested 1075 individual stick-insects; only 12 died. In the second type, ‘preference tests’, an individual on hatching was offered both ivy and privet over a period of several days until it showed a preference; this test was repeated twice more. Some individuals preferred privet, some ivy and others showed no preference.
The experiments began with stick-insects laying their eggs in autumn 1931 and continued until June 1937, the month of Miss Sladden’s death. The final of three papers was completed by H.R. Hewer who appears as co-author. MacBride added a footnote to the final paper which I cannot help but construe as patronising to both authors:
The tragic death of Miss Sladden occurred on 20 June 1937 when the final figures included in this paper were on the point of completion. The preparation of the manuscript is therefore solely the responsibility of Mr Hewer. It ought to be added that Mr Hewer deserves a considerable portion of the credit for Miss Sladden’s previous papers as he devised the type of experiment which she carried out.
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| H.R. Hewer |
Hewer was Humphrey Robert Hewer (1903-1974); a student at Imperial College he was Lecturer and Reader there from 1926 until 1964 when he was given a personal chair. From 1941 until 1945 he was Chief Rodent Officer at the Ministry of Food (there was a huge effort to control rats during the war). From 1967 until 1974 he was Chairman of the Farm Animals Welfare Committee for what was then the Ministry of Agriculture, Fisheries and Food (now DEFRA); in that role he had a public spat with W.H. Thorpe (see below) on the housing of poultry in battery cages. He is best known for his work on seals.
The first paper contains the results that are the most convincing since the experiments on the offspring of some of the privet-fed and all of the ivy-fed adults were done at the same time of year. I am using a more limited range of results (those from the second generation on privet only and from the second generation given ivy only) than those shown in the summary of the paper since it was shown subsequently that the season of the year had an effect on the tests done on privet-fed individuals.
31% of hatchlings of the privet-fed adults took ivy at the first presentation, 46% at the second. By contrast, 78% of those from ivy-fed adults ate ivy at the first presentation, 19% at the second. That was a clear demonstration of the food being eaten by the adult having an effect on the acceptability of food by the offspring.
Differences were also evident in the preference tests in that of the offspring of privet-fed parents, 44% preferred privet, 35% were neutral (no-preference) and 21% preferred ivy. By contrast, of the offspring of ivy-fed parents, 28% preferred privet, 35% were neutral and 35% preferred ivy.
Studies were continued into succeeding generations of privet- and ivy-fed stick-insects. Dorothy Sladden did an enormous amount of tedious organisational and observational work through six generations. With hindsight, it is easy to criticise an over-complicated experimental design and the fact that the various groupings of animals are not very well explained. More seriously, although she used large numbers of animals for the tests (50 or 100), some of the groups were derived from eggs produced by very few adults, those that had survived or been kept to adulthood, opening the possibility not of genetic selection but of bias in the epigenetic (the term used widely) variability of individuals. So instead of n=50, say, the true n was sometimes 3, or 4, or 6, or 8.
Later results were confounded by seasonal changes in the acceptability of ivy versus privet which could have been related to the physical and chemical composition of the leaves at different times of year. At first sight there is a curious finding. Even those fed privet throughout accepted ivy to a greater extent in each succeeding generation to an extent such that in the 6th generation, privet-fed and ivy-fed mothers were producing offspring that were virtually identical in their acceptance of ivy; it was only in the generations before this that there were differences between the ivy-fed and privet-fed. Hewer’s calculations in the final paper suggest that the seasonal changes were seen in the willingness of privet-fed but not in the ivy-fed adults to accept ivy.
The claim in the summary of second paper that experiments in which the insects for several generations were forced to accept a new food-plant (ivy) and then reared to maturity on it, have shown that the offspring of each subsequent generation accepted the ivy more readily than did their parents, is not supported by the data. From the second generation (the first from ivy-fed parents) the percentage of those hatchlings accepting ivy on the first presentation was 78; in succeeding generations it was 94, 90, 100 and 82. Because of the confounding effect of season and the marked changes in the acceptability of ivy to the offspring of privet-fed parents, it is perhaps better to interpret the results of a step-change in a single generation to the acceptability of the food plant the parents were eating.
A ‘reversal test’ was done in which the offspring of parents fed on ivy for three generations were offered privet; 95% accepted it on the first occasion. It is a pity that the persistence of the effect was not studied. For example, if the offspring of ivy-fed parents were fed privet, would the young from that generation readily accept ivy? Is there a critical period for the feeding of ivy, during egg-laying, for example. But these questions are with hindsight; in the climate the the day, I can see why Miss Sladden pressed on with just more generations and a reversal test.
Enquiries were also made in India on what might be the natural food plant of the species of stick-insect and of the preference for other species of plant. Poplar was consistently refused but rose, lilac and blackberry were more readily eaten than ivy or privet, while fuchsia was on a par with ivy and privet.
Hewer, who wrote the last paper, ended with:
It is therefore difficult to escape the conclusion that the forced ivy-feeding has induced the increased ability to accept ivy in succeeding generations in this parthenogenetic insect…
So that was it, an apparent Lamarckian effect and one that was seized on in no uncertain terms by MacBride, who could never be accused of understatement, when he wrote a memoir in The Times newspaper on 28 June 1937, the day after Dorothy Sladden’s death.
MISS DOROTHY SLADDEN
THE PROBLEM OF FUNCTIONAL INHERITANCE
Professor E. W. MacBride, F.R.S., writes:—
The recent death of Miss Dorothy Sladdcn in consequence of a motor accident was an irreparable loss to the science of experimental zoology. She was a trained investigator working in the aquarium of the Zoological Society, and supported by the Department of Scientific and Industrial Research. But the research on which she was engaged was no ordinary scientific problem; it was an attack on that most fundamental problem of life, the nature of inheritance.
This proof Miss Sladden set herself to provide. With indomitable perseverance and
marvellous skill she pursued this end: her ability to keep animals in good health and to induce them to breed in captivity was extraordinary. She loved and studied them all her short life; this power was one of those inborn gifts of genius which are bestowed on few. After years of patient study she triumphed, and two of her papers were published by the Royal Society; a third and finally conclusive one was nearly finished when a cruel fate snatched her away.
When the history of zoology comes to be written she will rank with McDougal [sic*] as one of the few who supplied the principle of functional inheritance with a basis of incontrovertible fact.
MacBride also referred to Dorothy Sladden in a letter to The Times of 7 May 1938 in a spat with R.A. Fisher. His customary—and completely erroneous—arguments were deployed and he ends:
The reaction to environment by constant repetition becomes slowly ingrained in the constitution of the stock was proved by my pupil, Miss Sladden, who was killed by a motor accident nearly two years ago, and to whose memory you devoted such a generous obituary notice in your columns. I suggest that Professor Fisher should study her papers in the Proceedings of the Royal Society…
MacBride was wrong; Dorothy Sladden’s work was not remembered ‘as one of the few who supplied the principle of functional inheritance with the basis of incontrovertible fact’. It has received the occasional mention in books on evolution and the epigenetic revolution.
When searching for places in which her work my have been cited I came across an online copy of the book written in 1952 by Philip Gilbert Fothergill (1908-1967), Historical Aspects of Organic Evolution; Fothergill was Senior Lecturer in Botany in what is now the University of Newcastle, then a constituent college of the University of Durham; it contained a description of Dorothy Sladden’s work although there was an error of interpretation and an incorrect table. I found the book a very strange read for 1952; it was written as if the modern synthesis had never happened. When I delved more, I found that a number of professional biologists had also not been impressed, although it has to be borne in mind that there was still a dichotomy of world views: Darwinian selectionist versus Lamarckist. Extracts from these reviews are in Wikipedia. A few were encouraging “the author has kept his personal opinions in the background and has considered the data without bias”; but others were damning: "there is no evidence presented [that] the author has understood or digested the great advances made in evolutionary theory during the past twenty years”; "biased by both the author's inclination to Lamarckism, and by his religious views”;”too uncritically accepting of Lamarckism, orthogenesis and John Christopher Willis' age and area hypothesis whilst ignoring evidence of their refutation”. Ernst Mayr wrote that the "book suffers from an overzealous belief in the inheritance of acquired characters [but] is most useful for a sympathetic account of neo-Lamarckians.” Fothergill, a native of Dumfries was elected to the Royal Society of Edinburgh in 1955. He wrote a number of articles for catholic magazines and newspapers in which he tried to explain bible stories in biological terms and how evolution was compatible with catholic beliefs. Fothergill was clearly not my sort of guy.
Dorothy Sladden’s work has been confirmed in insects, in the butterfly Pieris rapae**, for example.
Even soon after publication of her work, suggestions as the the non-genetic mechanism that might be involved in what would clearly fit into the category of maternal effect were appearing. For example, W.H. Thorpe in Cambridge suggested†:
But it seems worth considering whether the mechanism of pre-imaginal conditioning, such as has here been demonstrated, might not explain some of the results of Sladden and Hewer on the transference of the induced food habit of the stick insect Carausius morosus. If we assume that the new food plant, i.e. ivy, contains a chemical substance of particularly persistent and penetrating qualities, it might be possible to imagine enough of this substance being contained in the eggs to produce positive conditioning in the newly emerged nymph. This might be particularly the case in those insects in which the first food consists of the empty egg shell.
Julian Huxley initially had difficulties with the Sladden experiments in Evolution: The Modern Synthesis, first published in 1942:
There remain one or two results, such as that of Metalnikov (1924) on immunity in waxmoths, and of Sladden and Hewer (1938) on food-preferences in stick insects which seem prima facie to demand a lamarckian explanation (but see p. 303 n.). However, in view of the fate of other claims, and of the theoretical difficulties we shall discuss below, too much weight must not be attached to such isolated cases.
I have the third edition (1974) and I do not know whether the footnote on page 303 appeared in the first edition. It reads:
As Thorpe (1939) suggests, these results may also explain the interesting results obtained by Sladden and Hewer (1938) on the food-preferences of stick insects, for which, prior to Thorpe’s work, a lamarckian interpretation seemed almost inevitable. It will be of great interest to test Sladden’s results in the light of Thorpe’s methods, and with a species capable of sexual reproduction.
Huxley’s when he first wrote of Sladden’s results, was falling into that terrible trap of dismissing anything that did not fit what he interpreted as the explanation for the mechanism of evolution. Such a statement as, ‘too much weight must not be attached to such isolated cases’, diminishes, in my mind, Huxley as a scientist; the consensus of scientists is x; therefore, any other evidence can be ignored, was Huxley’s stance. Now where have I heard before?
Huxley does though demonstrate again the complete split in biological thought at the time: if there was a hint of transmission of a change in character of the parent to the offspring, it must be Lamarckian since only one pathway of heredity was known or considered, that of alteration to the germline.
However, by the time he wrote the footnote, the penny must have been dropping on the notion of transmission of useful information to the offspring by the mother that does not involve alterations to the germline. It is odd that in the main text, he deals with plasticity in relation to olfactory conditioning, and the limits to plasticity ‘hedged about by genetic safeguards’, but only later adds Thorpe’s (1939) interpretation of the Sladden results as, in different words, an epigenetic phenomenon, in a footnote.
Miss Sladden was working at the Zoo when Huxley was Secretary and he must surely have discussed the results with her before her death with or without the presence of MacBride who was on the Zoological Society’s Council but who had retired from Imperial College. But clearly, he had not considered any explanation of her findings other than a classic Lamarckian one when he wrote his book.
The sort of maternal effect that Dorothy Sladden found is analogous to the findings in human infants where the diet of the pregnant mother has been shown to affect the food preferences of the young. There was a general article in Science (345, 750-751) in 2014 on the topic although it failed to give credit to workers other than those mentioned.
Whether one calls such phenomena as adaptive parental epigenetic effects Lamarckian is open to debate. Some people do. I do not because to me Lamarckism was viewed as such an alternative to the modern synthesis and espoused with such wrong-headedness that it is a term which reflects a concept from a bygone age that has been eliminated by natural selection; the fittest concept has survived and can easily accommodate additions such as epigenetic mechanisms that have been uncovered (although some clues, like the one Dorothy Sladden discovered, were there) since the genesis of the modern synthesis more than 70 years ago.
These are the three papers:
Sladden, D.E. 1934. Transference of induced food-habit from parent to offspring. Part I. Proceedings of the Royal Society B 114, 441-449.
Sladden, D.E. 1935. Transference of induced food-habit from parent to offspring -II. Proceedings of the Royal Society B 119, 31-46.
Sladden, D.E. & Hewer, H.R.1938. Transference of induced food-habit from parent to offspring. III. Proceedings of the Royal Society B 126, 30-40.
The fourth and final part of this series will cover Dorothy Sladden’s brief period at London Zoo.
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**Hovanitz, W. & Chang, V.C. 1965. The alteration of host plant specificity in larvae of Pieris rapae by induction. Journal of Research on the Lepidoptera 4, 13-21.
†Thorpe, W.H. 1939. Further studies on pre-imaginal olfactory conditioning in insects. Proceedings of the Royal Society B 127, 424-433.
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