human
The question you ask is one of great ornithological importance and I believe has never been worked out, but I am absolutely afraid to ask any questions in the British Museum, as they jump at an idea and cut the ground from under the original man's feet. This I regret to say is my experience. I have been asked what does it matter who makes the discovery? I reply, 'Render unto Caesar, etc.' If you are going to work it out, keep it dark. The British Museum have not the necessary specimens--in this country I believe it is not known how the change takes place. I tried some years ago to work it out with live specimens, but failed because I could not get young birds. Now in answer to your question, my belief is that the young bird moults into the winter plumages direct and that this is changed into the full plumage in spring either by a spring moult or by a shedding of the tips of the feathers. This is private because it is theoretical, and for your private use to verify....
gpt
The skin differs in nothing from that of Phoca foetida. In the skull I observe that the nasal bones are conspicuously narrower than in typical specimens from the northern coasts. There is also a remarkable thinness of bone, a want of osseous substance; but it is impossible to say whether this is due to altered physical conditions or should be accounted for by the youth of the specimen, or whether it is an individual peculiarity.