"- taking heed of your elders and betters -"

"- yes, and taking heed of your elders and betters, then eventually the Great Trout -"

Clop

Clop

"Yes?" said one of the younger mayflies.

There was no reply.

"The Great Trout what?" said another mayfly, nervously.

They looked down at a series of expanding concentric rings on the water.

"The holy sign!" said a mayfly. "I remember being told about that! A Great Circle in the water! Thus shall be the sign of the Great Trout!"

The oldest of the young mayflies watched the water thoughtfully. It was beginning to realise that, as the most senior fly present, it now had the privilege of hovering closest to the surface.

"They say, " said the mayfly at the top of the zigzagging crowd, "that when the Great Trout comes for you, you go to a land flowing with... flowing with..."

Mayflies don't eat. It was at a loss. "Flowing with water, " it finished lamely.

"I wonder, " said the oldest mayfly.

"It must be really good there, " said the youngest.

"Oh? Why?"

" ‘Cos no-one ever wants to come back."

Whereas the oldest things on the Discworld were the famous Counting Pines, which grow right on the permanent snowline of the high Ramtop Mountains.

The Counting Pine is one of the few known examples of borrowed evolution.

Most species do their own evolving, making it up as they go along, which is the way Nature intended. And this is all very natural and organic and in tune with mysterious cycles of the cosmos, which believes that there's nothing like millions of years of really frustrating trial and error to give a species moral fibre and, in some cases, backbone.

This is probably fine from the species' point of view, but from the perspective of the actual individuals involved it can be a real pig, or at least a small pink root-eating reptile that might one day evolve into a real pig.