Vestigial structures

Struthers lived in an exciting time for biologists. Charles Darwin's ideas on the evolution of species by natural selection was still the subject of intense debate and there was a great need for hard evidence that species really did change with time. Vestigial, apparently functionless organs were offered as one kind of evidence. The argument went that such organs would have been functional in an ancient ancestor but with time, and with a change in lifestyle, the organs had become less and less useful. As their utility declined they became reduced in size and in the fullness of time they might be expected to disappear altogether.

Whales have a number of vestigial structures that would have been fully functional in their land-living ancestors, including the pelvic girdle, the hind limbs and the finger muscles. Whales were a Godsend to the Darwinians!

The forelimbs of whales, which are used for steering, are stiff and paddle like and the muscles of the fingers, although still present, are much reduced, largely non-contractile and act more in the fashion of ligaments.

The Zoology Museum houses the fore limbs of a blue whale that had been found dead in the North Sea off Aberdeen on June 27th 1871. Struthers arranged for it to be towed into Peterhead Bay and then dissected it at low tide as it lay among the rocks. Struthers' own drawing of the bones of the hand and the digital muscles is reproduced from his detailed account of the dissection which was published as:

Struthers, J. (1871) On some points in the anatomy of a great fin whale (Balaenoptera musculus). J. Anat. Physiol. VI. 107-125.

The anatomy of the hand of a blue whale showing the bones and digital muscles.

The main propulsive force in whales comes, not from the hind limbs, from the spinal musculature and the tail fluke. The hind limbs, although still present, have become much reduced in size, are fully enclosed within the skin and are invisible from the outside.

In 1881 Struthers published, On the Bones, Articulations, and Muscles of The Rudimentary Hind-Limb of the Greenland Right-Whale (Balaena mysticetus). J. Anat. and Physiol. XV: 141-321. In it he wrote:

'Nothing can be imagined more useless to the animal than rudiments of hind legs entirely buried beneath the skin of a whale, so that one is inclined to suspect that these structures must admit of some other interpretation. Yet, approaching the inquiry with the most skeptical determination, one cannot help being convinced, as the dissection goes on, that these rudiments [in the Right Whale] really are femur and tibia. The synovial capsule representing the knee-joint was too evident to be overlooked. An acetabular cartilage, synovial cavity, and head of femur, together represent the hip-joint. Attached to this femur is an apparatus
of constant and strong ligaments, permitting and restraining movements in certain directions; and muscles are present, some passing to the femur from distant parts, some proceeding immediately from the pelvic bone to the femur, by which movements of the thigh-bone are performed; and these ligaments and muscles present abundant instances of exact and interesting adaptation. But the movements of the femur are extremely limited, and in two of these whales the hip-joint as firmly anchylosed, in one of them on one side, in the other on both sides, without trace of disease, showing that these movements may be dispensed with. The function point of view fails to account for the presence of a femur in addition to processes from the pelvic bone. Altogether, these hind legs in this whale present for contemplation a most interesting instance of those significant parts in an animal -- rudimentary structures'.

The hind-limb bones that Struthers dissected from the right whale are still on display in the Zoology Museum at Aberdeen.

The vestigial pelvis and hind limbs of a Greenland right whale