"The fifth is ambition. It next will be right
To describe each particular batch:
Distinguishing those that have feathers, and bite,
And those that have whiskers, and scratch.
"For, although common Snarks do no manner of harm,
Yet, I feel it my duty to say,
Some are Boojums--" The Bellman broke off in alarm,
For the Baker had fainted away.
Missing Link feathers and bite = bird is the common harmless Snark; whiskers and scratch = mammal is the dangerous Boojum. It is the search for the Common Ancestor of both birds and mammals.
Now we know that Birds are the grandchildern of Dinosaurs, the Common Ancestor was 100 Million BC.
Boojum made Baker disappear. Many assume that Boojum makes everyone disappear. My guess is that Boojum does Room 101 and Boggart: it does your greatest fear. Disappearing was just Baker's fear.