Embracing diversity

I saw these two snails getting it on in the morning dampness  (I didn't put them like this.)

They are common banded snails,  Cepaea nemoralis, that make up a large part of the diet of thrushes on whose anvils the shells can often be seen. They come in three versions: brown striped, yellow striped and plain yellow. Their genetics have been studied and it appears that the colour variations give each type an advantage depending on environmental and seasonal variations: the brown ones show up less in the autumn against a background of dried grass and dead leaves while the yellow ones merge  better into verdant spring surroundings. 

I left them to it and they had gone when I next looked. The snails themselves make no odds about  external appearances.