Target cells that are known to be having specific receptor for an antigent or antibody and the main reason why all cells are not target cells for the hormones is that they act as the focus for contact by phagocyte.
Answer: As altitude increases, the spacing between particles increase.
Explanation: There is less gravity when altitude increases to compact the particles together and particles are more free flowing to move around. Therefore the particles aren't as close together, increasing the amount of space in between them.
It was during the Paleozoic Era that plants (first known from microfossils called cryptospores that appear in the mid-Ordovician, about 470 million years ago; Wellman and Gray 2000) and animals (known from Silurian fossils, at least 423 million years ago; Wilson and Anderson 2004) began to colonize the land
