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Across 3. if no members of a species is still alive, then they are... 5. type of rock that fossils typically form in 6. remains of organisms from the past 7. a group of tissues that act together to perform a function 8. the splitting of one species into two over time 11. structures that related organisms share (we colored them in class) 15. structures or behaviors that help organisms survive and reproduce in their environments 17. any difference between individuals of the same species 19. sections of DNA that code for proteins or traits on chromosomes

Down 1. developed to prevent people from getting infections such as small pox and measles 2. forming more offspring than will survive in an environment 4. the study of organisms in their early stages of development 9. theory that states that all species have changed over time 10. this time scale divides Earth's history into eras and periods 12. scientist associated with the idea of acquired characteristics 13. a population of organisms that are able to interbreed and form fertile offspring 14. nonliving particles that need host cells to reproduce 16. wrote "The Origin of Species" published in 1859 18. an explanation in science that has been proven accurate multiple times