d fourth toes. Now, in the wing of the pigeon, or any other bird, the
first and fifth digits are wholly aborted; the second is rudimentary, and
carries the so-called 'bastard wing;' whilst the third and fourth {182}
digits are completely united and enclosed by skin, together forming the
extremity of the wing. So that 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
connexion of feathered legs and membrane between the outer toes."

Irregularities in the circulating system are far from uncommon, and
sometimes illustrate this homological tendency. My friend and colleague Mr.
George G. Gascoyen, assistant surgeon at St. Mary's Hospital, has supplied
me with two instances of symmetrical affections which have come under his
observation.

In the first of these the brachial artery bifurcated almost at its origin,
the two halves re-uniting at the elbow-joint, and then dividing into the
radial and ulnar arteries in the usual manner. In the second case an
aberrant artery was given off from the radial side of the brachial artery,
again almost at its origin. This aberrant artery anastomosed below the
elbow-joint with the radial side of the radial artery. In each of these
cases the right and left sides varied in precisely the same manner.

Thirdly, as to pathology. Mr. James Paget,[190] speaking of symmetrical
diseases, says: "A certain morbid change of structure on one side of the
body is repeated in the exactly corresponding part of the other side." He
then quotes and figures a diseased lion's pelvis from the College of
Surgeons Museum, and says of it: "Multiform as the pattern is in which the
new bone, the product of some disease comparable with a human rheumatism,
is deposited--a pattern more complex and irregular than the spots upon a
map--there is not one spot or line on one side which is not represented, as
exactly as it would be in a mirror, on the other. The likeness has more
than daguerreotype exactness." He goes on to observe: "I need not {183}
describe many examples of such diseases. Any out-patients' room will
furnish abundant instances of exact symmetry in the eruptions of eczema,
lepra, and psoriasis; in the deformities of chronic rheu