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Basically because your skin has several layers to it (see link, below), and the layer that has the blood vessels capable of bleeding in it is further down than a superficial scrape would affect.
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The reason is that something changed that caused them to be unsuccessful in their environment.
Sorry if it is not right.
Answer:
The number of double bonds in the fatty acid chain
Homogeneous chromosoms pair up
<span>Positive feedback</span><span>
There are two types of feedback mechanisms
Positive feedback heightens the intensity
Negative feedback lowers the intensity
These all serve to maintain homeostasis. Homeostasis is the state where the internal and external part of the body maintains and establishes balance and equilibrium. </span>
