Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!turpin
From: turpin@cs.utexas.edu (Russell Turpin)
Newsgroups: sci.bio
Subject: Re: What distinguishes a species?
Summary: So what *are* the dog species ...
Message-ID: <7757@cs.utexas.edu>
Date: 1 Feb 90 21:14:27 GMT
References: <22448@siemens.siemens.com> <202@53iss6.Waterloo.NCR.COM>
Distribution: usa
Organization: U. Texas CS Dept., Austin, Texas
Lines: 15

In article <202@53iss6.Waterloo.NCR.COM>, howard@53iss6.Waterloo.NCR.COM (Howard Steel) writes:
>> Are all dogs in the same species?
> 
> NO. The main problem is usually physical disparity. When's the last time you
> saw a St. Bernard - Chihuahua cross?

But a St Bernard will happily breed with a large shepherd, which
will breed with a medium collie, which will breed with a smallish
terrier, which will breed with the Chihuahua.  If one uses solely
the above criterion for being in the same species, one has the
problem that the relation "being in the same species" is no longer
transitive.  Hence, the concept of a species (as a group) 
disappears, to be replaced by the comparative but non-transitive
relation of breedability.

Russell