1. natural selection acts on existing traits because you can not evolve into something you didn't have
2. original structures evolve to accomplish new function in order to survive
Answer: The cell is the structural and functional unit of all living organisms, and is sometimes called <u>the "building block of life."</u> Some organisms, such as bacteria, are unicellular, consisting of a single cell.
<span>1.keeps tooth alive
</span><span>pulp
2.surface of teeth
enamel
3.tube carrying food to stomach
esophagus
4.last major organ in digestive system
large intestine
5.digestive organ located above waist, close to ribs
stomach
6.first stage of digestion takes place here
mouth
7.regular squeezing movements
peristatiss
8.blood vessel leading from heart
artery
9. windpipe
trachea</span>