The picture shows 10 samples from a sequence of snail shells, oldest on the left, youngest on the right.
If there had been gaps in the fossil sequence, we would have thought that these were fossils from several different species. If we look at snails alive today, we can find separate species which differ by less than the difference shown in the picture.
The picture was scanned from
Life, The Science of Biology, Second Edition, Purves and Orians, Sinauer Associates 1987, page 1041The shells are from a freshwater deposit in Yugoslavia, laid down from 10 million years ago to 3 million years ago.