What led 17th century scientists to believe that at one time a super-continent, Pangaea, existed? the observation that the continents fit together like a jigsaw puzzle
the presence of mid-ocean ridges
the discovery of the composition of the asthenosphere
the location of seas at the boundaries of continents

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

a) carbon, hydrogen

b) carbon, hydrogen, oxygen, nitrogen

c) carbon, hydrogen, oxygen, nitrogen, phosphorus

d) sugar

e) amino acid

f) nucleotide