Morphospace.

In his famous (if uncharacteristic) burst of lyricism at the end of the Origin Darwin described biodiversity as "endless forms most beautiful and wonderful". It is easy to agree with him when one considers red-lipped batfish or pelagic holothurians. But are they endless, or are there limitations to the variety of forms - and if there are, where do they come from? Can morphological evolution be described by Brownian motion of a gas, slowly diffusing to fill up all the space of possible forms, or does it operate within a certain set of constraints? And if there are constraints, where do they come from? The concept of morphospace is an attempt to map out the products of evolution within a quantitative framework to try to shed light on these questions.