Darwin begins his discussion by admitting contemporary ignorance about causes of variation: “I have hitherto sometimes spoken as if the variations — so common and multiform in organic beings under domestication, and in a lesser degree in those in the state of nature — had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation” (1859, p. 131).
When we do not know the underlying causal bases of important and related phenomena, taxonomies based upon overt expressions become our best practical procedure for specification and understanding. Darwin therefore tries to gather the phenomena of variation into categories. Most categories either enhance adaptation by routes other than natural selection (use and disuse, and direct effects of the environment — Lamarckism and Geoffroyism in later parlance), or, at most, serve to enhance or slow down selection by affecting the amount of available raw material in variation. Only one category truly challenges the functionalist credo by embracing the primary structuralist theme of internal constraint upon adaptation, with consequential no functionality for certain features. Darwin names this category “correlations of growth,” and offers a definition: “I mean by this expression that the whole organization 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. This is a very important subject, most imperfectly understood” (1859, p. 143). Note, even here, how Darwin defends the primacy of selection (by the “sequelae” argument [Page 334] of the relative frequency tradition — see pp. 255–256). Natural selection builds a feature, and others follow by correlational linkage to this generating cause.
Darwin clearly defines correlation of growth as a category contrary to natural selection, for he explicitly excludes the common case of taxonomically correlated structures that arise by separate selection on each feature, with later joint propagation by simple inheritance (wings and beaks in birds). He defines correlation of growth, on the other hand, as structurally forced association independent of immediate selection: “We may often falsely attribute to correlation of growth, structures which are common to whole groups of species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired through natural selection some one modification in structure, and, after thousands of generations, some other and independent modification; and these two modifications, having been transmitted to a whole group of descendants with diverse habits, would necessarily be thought to be correlated in some necessary manner” (1859, p. 146).
Darwin's genuine interest in correlations of growth arose from several sources, including the mystery surrounding the subject. “The nature of the bond of correlation is very frequently quite obscure . . . What can be more singular than the relation between blue eyes and deafness in cats, and the tortoise-shell color with the female sex?” (1859, p. 144). Again, as for the larger issue of variation in general, Darwin follows a fruitful operational strategy: when you can't specify causes, at least gather overt phenomena into reasonable categories. The short epitome of the Origin (where correlation of growth receives seven pages of discussion) lists four major categories:
1. Adaptive modifications of early ontogeny that propagate effects into later growth: “The most obvious case is, that modifications accumulated solely for the good of the young or larvae, will, it may safely be concluded, affect the structure of the adult; in the same manner as any malconformation affecting the early embryo, seriously affects the whole organization of the adult” (1859, p. 143).
2. Correlated variation in serially homologous and symmetrical structures of the body. (Note how Darwin, following the popular structuralist theory of the vertebral archetype, viewed correlated variation of jaws and limbs as potentially homologous): “The several parts of the body which are homologous, and which, at an early embryonic period, are alike, seem liable to vary in an allied manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed to be homologous with the limbs” (1859, p. 143).
Although he cited the argument of ontogenetic correlation as first and most obvious, Darwin devoted more interest and attention to this second category of homologous variation. He presents (especially in the longer version of 1868 — see pp. 336–341) a variety of wide-ranging and intriguing cases (not all correct, of course). For example, in regarding teeth and hair as homologous, Darwin conjectures: “I think it can hardly be accidental, that if we pick [Page 335] out the two orders of Mammalia which are most abnormal in their dermal covering, viz. Cetacea (whales) and Edentata (armadilloes, scaly anteaters, etc.), that these are likewise the most abnormal in their teeth” (1859, p. 144).
Nonetheless, since Darwin remains eager to assert the primacy of natural selection as the centerpiece of his worldview, he reminds us that correlation can only be subsidiary in impact — ever present to be sure, but always subject to cancellation if selection favors dissociation: “These tendencies, I do not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed with an antler only on one side; and if this had been of any great use to the breed it might probably have been rendered permanent by natural selection” (1859, p. 143).
3. Homologous parts not only vary together, but also tend to join or fuse. “Homologous parts, as has been remarked by some authors, tend to cohere ... nothing is more common than the union of homologous parts in normal structures, as the union of the petals of the corolla into a tube” (1859, pp. 143-144).
4. One part (usually hard upon soft) may impress its form upon another: “Hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys. Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child” (1859, p. 144).
Darwin considered one further category, strongly emphasized by Goethe and, later, by Geoffroy as the “Loi de balancement” or compensation: “If nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to give much milk and to fatten readily. The same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil-bearing seeds” (p. 147). But Darwin, while acknowledging the importance and intellectual pedigree of this principle, wisely chose to exclude compensation from his discussion of structural correlation because he could state no clear criterion (and the problem remains just as vexatious today) for separating negative interaction due to selection from forced and nonadaptive correlation due to limited resources: “For I hardly see any way of distinguishing between the effects, on the one hand, of a part being largely developed through natural selection and another and adjoining part being reduced by this same process or by disuse, and, on the other hand, the actual withdrawal of nutriment from one part owing to the excess of growth in another and adjoining part” (p. 147).
Although limited space and numerous hedges clearly indicate the subordinate status of constraint to adaptation in Darwin's evolutionary views, he evidently did take serious interest in correlations of growth, and he did identify the theme as contrary to, or at least independent of, natural selection — as in this statement: “I know of no case better adapted to show the importance of the laws of correlation in modifying important structures, independently of utility, and therefore, of natural selection, than that of the difference between the outer and inner flowers in some Compositous and Umbelliferous plants” [Page 336] (1859, p. 144). (Note Darwin's interesting linguistic choice in designating this example as so well “adapted” to illustrate constraint.)
Darwin also allowed that taxonomically important, and not just trivial, characters co
uld be shaped by correlations of growth: “Hence we see that modifications of structure, viewed by systematists as of high value, may be wholly due to unknown laws of correlated growth, and without being, as far as we can see, of the slightest service to the species” (1859, p. 146).
Darwin, as all professional evolutionists know, had been writing a much fuller version of his evolutionary views (a book that would have been about as long as Lyell's three-volume Principles of Geology) when Wallace's note from Ternate arrived in 1858. The hurried Origin of Species (1859) is an epitome (of 490 pages!) without formal references. Darwin intended to complete and publish the longer version, but never realized this project. Instead, he took most of the material designated for the early part of Natural Selection (Darwin's putative title for the full treatment), expanded his coverage, and published his longest book in 1868, the two-volume Variation of Animals and Plants Under Domestication. (Incidentally, the fourteen-page introductory chapter to volume 1 presents Darwin's clearest and most cogent general summary of his evolutionary views and methodological principles. This largely unread essay should be assigned to all students of evolution.)
Volume one provides a chapter-by-chapter treatment of various domesticated species (with Chapter 5 on “domestic pigeons,” unsurprisingly given Darwin's interest, as the most illuminating). This material largely represents an expansion of Chapter 1 in the Origin, “Variation under domestication.” Volume two then presents Darwin's general ideas on variation and inheritance (an expansion of Chapters 4 and 5 of the Origin, with much added material). Darwin actually published more about inheritance than about natural selection, thus refuting the common statement that he neglected the subject of heredity — a myth engendered by the retrospective fallacy, given his success with selection and his limited impact in resolving the principles of heredity. Volume two includes sets of chapters on four subjects: inheritance, crossing, selection, and causes of variation, all capped with the chapter that would become (along with his geological error on the “parallel roads” of Glen Roy) his nemesis — Chapter 27 on “the provisional hypothesis of pan-genesis” (Darwin's Lamarckian conjecture about the nature of inheritance). Darwin included his material on structural constraint within this volume, providing an expanded version of his views in Chapters 25 and 26 on “correlated variability,” a term that he now regards as preferable to “the somewhat vague expression of correlation of growth” (1868, vol. 2, p. 319), used previously.
However, despite extensive elaboration and addition of examples, Darwin's treatment scarcely differs from the short version in the Origin. He presents the same taxonomy for modes of “correlation of growth.” Again, early modification with propagating effects through ontogeny wins first place, although Darwin now adds some interesting examples: “with short-muzzled races of the dog certain histological changes in the basal elements of the [Page 337] bones arrest their development and shorten them, and this affects the position of the subsequently developed molar teeth” (1868, vol. 2, p. 321).
Darwin then inserts a new category, strangely overlooked in 1859 (a testimony, perhaps, to his subsidiary concern for the general subject of structural constraint) — allometric effects of change in size, either of the whole body, or of parts:
Another simple case of correlation is that with the increased or decreased dimensions of the whole body, or of any particular part, certain organs are increased or diminished in number, or are otherwise modified. Thus pigeon-fanciers have gone on selecting pouters for length of body, and we have seen that their vertebrae are generally increased in number, and their ribs in breadth.... In Germany it has been observed that the period of gestation is longer in large-sized than in small-sized breeds of cattle. With our highly improved animals of all kinds the period of maturity has advanced . . . and, in correspondence with this, the teeth are now developed earlier than formerly, so that, to the surprise of agriculturists, the ancient rules for judging the age of an animal by the state of its teeth are no longer trustworthy (1868, vol. 2, p. 321).
As in 1859, Darwin devotes most attention to correlated variability in homologous structures. In part, this preference can claim a methodological basis, for Darwin finds less mystery in the bonding or joint variation of homologs than in most other cases of structural constraint:
In many cases of slight deviations of structure as well as of grave monstrosities, we cannot even conjecture what is the nature of the bond of connection. But between homologous parts — between the fore and hind limbs — between the hair, hooves, horns, and teeth — we can see that parts which are closely similar during their early development, and which are exposed to similar conditions, would be liable to be modified in the same manner. Homologous parts, from having the same nature, are apt to blend together, and, when many exist, to vary in number (1868, vol. 2, pp. 419-420).
Amidst a plethora of interesting examples, Darwin cites pigeons that develop feathers and incipient wing-like membranes on portions of the foot corresponding to the position of wings on the forelimbs: “In feather-footed pigeons, not only does the exterior surface support a row of long feathers, like wing feathers, but the very same digits which in the wing are completely united by skin become partially united by skin in the feet; and thus by the law of the correlated variation of homologous parts we can understand the curious connection of feathered legs and membrane between the two outer toes” (1868, vol. 2, p. 323).
Citing the theory of vertebral archetypes again, Darwin tries to correlate variation in head and limb bones, while acknowledging that not all biologists accept the idea: “If those naturalists are correct who maintain that the jawbones are homologous with the limb bones, then we can understand why the [Page 338] head and limbs tend to vary together in shape and even in color; but several highly competent judges dispute the correctness of this view” (1868, vol. 2, p. 324). He also argues that joint slimness in head and limbs of greyhounds, and similar thickness of both structures in draft horses, might record structural correlation between putative homologs.
Proceeding further, Darwin considers hair, feathers, hooves, horns and teeth as “homologous over the whole body” (p. 326). He argues that sheep with a tendency to grow multiple horns also have “great length and coarseness of fleece.” He also reports that sheep with more curly wool bear more spirally twisted horns, and that many breeds of hairless dogs grow deficient teeth.
Moving to humans, Darwin tries to relate some forms of inherited baldness to weakness of dentition, and even claims that rare cases of restored hair in old age may be accompanied by renewal of teeth. In his most intriguing example, Darwin discusses Julia Pastrana, the famous “bearded lady” of circus sideshows. Proclaiming her, with true Victorian sensibility, “a remarkably fine woman” (p. 328), Darwin continues: “... but she had a thick masculine beard and a hairy forehead; she was photographed, and her stuffed skin was exhibited as a show; but what concerns us is, that she had in both the upper and lower jaw an irregular double set of teeth, one row being placed within the other . .. From the redundancy of the teeth her mouth projected, and her face had a gorilla-like appearance” (p. 328).
Darwin even invokes putative homology — this time between organs of sight and hearing — to explain the case that he had always found personally most bothersome: deafness correlated with blue eyes in cats:
The organs of sight and hearing are generally admitted to be homologous, both with each other and with the various dermal appendages; hence these parts are liable to be abnormally affected in conjunction ... Here is a more curious case: white cats if they have blue eyes, are almost always deaf . . . This case of correlation in cats has struck many persons as marvelous ... [But,] we have already seen that the organs of sight and hearing are often simultaneously affected. In the present instance, the cause probably lies in a slight arrest of development in the nervous system in connection with the sense organs. Kittens during the first nine days, whilst their eyes are closed, appear to be completely d
eaf; I have made a great clanging noise with a poker and shovel close to their heads, both when they were asleep and awake, without producing any effect. ... Now, as long as the eyes continue closed, the iris is no doubt blue, for in all the kittens which I have seen this color remains for some time after the eyelids open. Hence if we suppose the development of the organs of sight and hearing to be arrested at the stage of the closed eyelids, the eyes would remain permanently blue and the ears would be incapable of perceiving sound; and we should thus understand this curious case (pp. 328-329). [Page 339]
The Structure of Evolutionary Theory Page 54
