Clades are not classes

[SKÁLA Zdenek]

I will try to summarize a bit:

>pierre:
>A snake is a tetrapod. Not in the data matrix (it has no legs), but as
>interpreted once the phylogeny is accepted ...
>"Lost legs" is a confirmed synapomorphy among tetrapods.
...
>If the physical presence of features is a requirement of the class theory, 
>of course all tetrapods do not share the presence of legs and tetrapods is 
>not a class of equivalence.
>It can be one only if "lost legs" (reversal) can be accounted as a common 
>property of snakes and other tetrapods (having four legs, present or 
>secondarily lost).

Snakes are members of tetrapods as a *clade* and four legs are perfectly a property of tetrapods as a clade. However, it does not mean that all members of this clade share this property (not even in some "cryptic" form); clade is an entity of its own and can have properties that are not shared by all members (say "plesiomorphic status" of the clade, in theory). On the other hand, sharing of some property is necessary for a group to be a class. This is why tetrapods are a clade but not a class.
NB: All what can be said by cladistic analysis (of reversals) is that the reversal is a new transformation event and in this respect the reversed character state is independent of the plesiomorphic character state (no matter that superficially identical). This is OK; problematic is the statement that the reversed character state preserves in some respect the earlier (apomorphic) character state. I see no reason to assume this (and cladistic analysis does not need it - homoplasy was already resolved as reversal).
Best!
Zdenek