Last Saturday afternoon while on a walk near my house, I noticed a shiny track on the sidewalk. It ended at a shriveled object stuck to the hot, dry concrete and hardly resembling a slug.
The slug had probably come out of its hiding place during the night, perhaps when the sidewalk was still wet from the Friday's rain. Then, after the sun rose and things started to get hot, it tried to get away. It was going in the right direction when it ran out of slime just six centimeters short of the grassy field along the sidewalk.
I pressed my fingers on the soil among the grass roots and felt that it was still damp at that hour. Six more centimeters, and the slug would have reached safety.
A snail would have survived. It would have withdrawn into its shell and, provided that it didn't get stepped on, waited until the next rain or the night time when the humidity rose before coming out again. Although at times like this it is good to have a shell, at other times being a slug has its advantages too.
A long time ago, when the snail-like ancestors of slugs started out on the road towards slugdom they began to make smaller and smaller shells. This way, they were able to divert resources from shell making to growth and reproduction. They could grow big and lay big eggs without having to also build big shells. They also became faster, for they didn't have to carry heavy, cumbersome shells on their backs. Moreover, when it got too hot or dry, or when there were predators around, they could squeeze into the nooks and crannies that the snails, because of their shells, wouldn't fit. And they didn't have to cross any hot sidewalks.
Millions of years later on a suburban sidewalk, one of their descendents had to pay the price.