Here in the UK we have two sorts of gulls (amongst others) one is called the black headed - because in the breeding season it has a black head - the other has a black back. They are about the same size and shape, but they are regarded as separate species as they do not interbreed.

If you leave these shores and proceed around the world - either way - your choice - you will find that other places have a similar gull - but only one species, not two.

If you keep going you will see that there are gulls very like the ones in the place you just left - but - as you go round the world the one sort slowly changes into the other. If you were to take gulls from a one gull species spot to another a few hundred miles further on in the circumnavigation they would have little trouble integrating with the locals. It is only when you reach here again that the sum of all the differences is apparent.

The UK has both ends of the spectrum, or series, whatever you want to call it. It seems to show that what we call a species is just the end of a very long line.

Anne