Gijs C. Kronenberg pointed out that my discussion of Dall's types was misleading.  Exactly what Dall did is not fully clear, and I do not have the references on hand to confirm.  If a single specimen is designated in the original description, it is the holotype for the species.  If multiple specimens are designated, they are syntypes.  A later designation of a single specimen from among the syntypes makes it a lectotype.

In the case of Dall's specimens, the designation in the original was not very clear as far as I recall.  In a later catalog of the type specimens at the USNM, he recorded some specimens as types without explicitly stating that he was making new designations.  In his defense, some of the relevant taxonomic rules were not yet in place.

As to why five digits prevails among vertebrates, they are to help you count your shell collection.  The non-shell related evolutionary explanation is that the early amphibians seem to have adopted that number (reduced from the very first amphibians) and little variation has followed, mostly reduction (in caecilians, many salamanders, many reptiles, all birds, and many mammals).  The number of digits is set relatively early in development, and messing with early developmental pathways is likely to have side effects, which may account for the rarity of major changes.

A similar phenomenon in mollusks comes from the pattern of larval development.  If a mutation occurs leading to a shortened larval period, the chance of reversing it seems relatively slim.  This also would tend to promote the evolution of local species from an initially widespread stock.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!