What common theme do the missing elements have?
Group 1
Group 2
Group 3
Group 4
Image credits:
1.3 Hex, CC-BY-SA 3.0
2.1 Nevit Dilmen, CC-BY-SA 3.0
2.2 Cramyourspam, CC-BY-SA 4.0
3.2 Jens Buurgaard Nielsen, CC-BY-SA 3.0
4.2 Saffron Blaze, CC-BY-SA 3.0
Others PD/CC0