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Darwin Correspondence Project

Correlation of growth: deaf blue-eyed cats, pigs, and poison

Deaf Cat

Our cats and all about them: their varieties, habits, and management, and for show, the standard of excellence and beauty by Harrison Weir, p. 17
Our cats and all about them: their varieties, habits, and management, and for show, the standard of excellence and beauty by Harrison Weir, p. 17 (Tunbridge Wells: R. Clements and Co., 1889)
Biodiversity Heritage Library


Darwin made many changes to the text of Origin across different editions as he responded to criticisms, or received new information, or simply tried to make his meaning clearer. Although he is more often associated with dogs than cats, it was cats that led to one such long-running set of revisions (he even ended up as patron of the Crystal Palace cat shows).

As he was first developing his ideas, among the potential problems Darwin recognised with natural selection was how to account for developmental change that conferred no apparent advantage.  He proposed a ‘mysterious law’ of ‘correlation of growth’ where advantageous changes in one part of an organism dragged associated, but otherwise useless, changes in other parts along in their wake.  The chapter on ‘Laws of variation’ in the ‘big book’ he was writing in the 1850s, included a section on the correlation of growth (Natural selection, pp. 297–304). ‘I mean by this expression’ he later explained in Origin (p.143), ‘that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified.’

He set about looking for ‘Cases of odd & inexplicable connection, between different parts of structure, so that if one changes the other changes’.  He had particularly in mind otherwise inexplicable changes of colour.

One such apparent example of correlation in seemingly unrelated characteristics, was an association between deafness in cats, and the colour of their fur and eyes. 


Are white cats with blue eyes deaf?

That there might be such a correlation seems to have been first brought to Darwin’s notice by his cousin, William Darwin Fox.  As Darwin was beginning to write up a ‘preliminary essay’ on his views in 1856, he went back to Fox to check his facts, asking if he could include an assurance from “the Rev. W. D. Fox that he has never seen or heard of a blueish-grey Cat which was not deaf.” Fox presumably put him straight that it was the colour of the eyes and not the fur, and in the first edition of Origin, Darwin stated that ‘cats with blue eyes’ were ‘invariably deaf’ (Origin, p. 12; see also p. 144). Moreover, although he didn’t say so in print, he had been told that cats with only one blue eye were deaf in only one ear.

The case was not water-tight however, and by the third edition of Origin, Darwin had already diluted the claim to: ‘cats with blue eyes are generally deaf’ (Origin 3d ed., p. 12).  He had refined it yet further by the 4th edition to only blue-eyed cats which were ‘entirely white’.  That was a distinction already pointed out to him by October 1860, by which time he was in correspondence with an old Cambridge friend, John Medows Rodwell, who had offered him information about his own white cat.  Rodwell had bad news: not only was he certain his own white, blue-eyed cat was not ‘in the slightest degree deaf’, but a survey of his neighbours found other examples.  A ‘Mr Bedingfield’, ‘proprietor of a well known horticultural & fishing rendezvous for Cockneys’ had five white cats and a kitten, as well as four black cats, and all ‘9 ½’ cats could hear the dinner call.  Helpful readers also sent counter examples indirectly through the letters pages of The Field.

By the time he came to discuss the phenomenon in Variation, Darwin was forced to acknowledge that although he ‘formerly thought that the rule was invariable,’ had now heard of ‘a few authentic exceptions’ (Variation 2: 329), and still they kept coming.  By this stage he had also replaced the term ‘correlation of growth’ with ‘correlated variability’, first introduced in Variation and substituted for ‘correlation of growth’ in Origin 5th and 6th editions.  As he explained (Variation 2: 319–38), he had formerly used the ‘somewhat vague expression’ of ‘correlation of growth’, which could too easily be broadened to include a wide range of associated characteristics (as critics – and he named George Campbell, the Duke of Argyll as one – had done), and he sought to distinguish the particular case of ‘correlated variability’ from what in a letter to another critic, George Maw, he calls ‘accidental correlation’.  ‘In cases of true correlated variation,’ he went on, ‘we are sometimes able to see the nature of the connexion; but in most cases the bond is hidden from us, and certainly differs in different cases. . . .  Correlated variation is an important subject for us; for when one part is modified through continued selection, either by man or under nature, other parts of the organisation will be unavoidably modified.’

In a further complication to the example of cats, one observer, Lawson Tait, later claimed that it was only male blue-eyed cats that were hard of hearing. Darwin mentioned Tait’s assertion in the 6th edition of Origin (p. 9), but a few years later, as he was putting the finishing touches to the second edition of Variation, he went back to Fox to ask specifically about the sex of his deaf white cats. Fox, possibly a little exasperated to be quizzed again, wrote a long and detailed reply asserting that he had observed ‘not one dozen, but dozens of white cats’ (letter originally tentatively dated 1860, and now redated 1875).  Anecdotes about the Fox family cats may have been too imprecise for Darwin, but Fox also described two deaf white Norwegian cats that were definitely female, possibly the two examples Darwin refers to in the book (Variation 2d, 2: 322 n. 24).  By this stage Lawson Tait was a frequent and, as Darwin told Joseph Hooker, an increasingly bothersome correspondent, prone, Darwin suspected, to absurd ideas.


Pigs and poison

Darwin had never relied solely on this one example to illustrate correlation.  His go-to list from the first edition of Origin onwards included sheep, pigs, dogs, and pigeons:

white sheep and pigs are differently affected from coloured individuals by certain vegetable poisons. Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet’ (Origin p. 12).

While he had first-hand knowledge of pigeons and dogs, Darwin felt on shakier ground with sheep and pigs, and he was relieved to be able to include in the third edition of Origin a detailed case, sent to him by Jeffries Wyman, of black pigs being immune to the effects of specific poisonous plants:

Professor Wyman has recently communicated to me a good illustration of this fact; on asking some farmers in Florida how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the "crackers" (i.e. Florida squatters) added, "we select the black members of a litter for raising, as they alone have a good chance of living." (Origin 3d ed. p. 12).

‘I have been the more glad to get your Hog case,’ Darwin confided to Wyman, ‘as I was hardly able to credit the parallel case of sheep in Sicily.’  And Wyman’s case was ‘a marvellous relation of mere colour, (generally thought to be so unimportant) & constitution’.


Human races

Darwin suspected that ‘correlation of growth’ could also explain racial differences in skin colour, but he was never able to gather sufficient evidence to make the case. He wrote ‘several letters to various parts of the world’ asking for information – a long but unsatisfactory letter from the African explorer and army surgeon William Daniell in 1856 was probably in reply to such a request – and told Asa Gray in 1859 that he still wanted information on yellow fever among Europeans of differing skin colour. He suspected a correlation between paler skin and susceptibility to tropical diseases, but as he lamented to Wyman he could get ‘no facts to support this crude speculation’. He later tried to gather data by mobilising army doctors in the British colonies. In 1862 he got permission to send out a questionnaire, but got no responses.  

Darwin remained hopeful, using the pages of Descent to advertise his request:

the following investigation seems worth consideration. Namely, whether there is any relation in Europeans between the colour of their hair, and their liability to the diseases of tropical countries. If the surgeons of the several regiments, when stationed in unhealthy tropical districts, would be so good as first to count, as a standard of comparison, how many men, in the force whence the sick are drawn, have dark and light-coloured hair, and hair of intermediate or doubtful tints; and if a similar account were kept by the same medical gentlemen, of all the men who suffered from malarious and yellow fevers, or from dysentery, it would soon be apparent, after some thousand cases had been tabulated, whether there exists any relation between the colour of the hair and constitutional liability to tropical diseases. . . . Theoretically the result would be of high interest, as indicating one means by which a race of men inhabiting from a remote period an unhealthy tropical climate, might have become dark-coloured by the better preservation of dark-haired or dark-complexioned individuals during a long succession of generations. (Descent 1: 244–5 n. 48)