Natural Selection.
An easy and important way to remember this is by thinking of a species - let’s say a bright white moth. For ages, these moths have survived beautifully, matching perfectly with the white tree bark they live on, until one day, a smoky building begins pumping its soot into the air. This air begins to change the color of the tree bark to black and the once hidden white moths are now plainly visible to birds who eat them easily. Fortunately, every now and then a moth is born who is darker than the rest - black as soot even. And so, the birds keep eating the white moths but missing the soot-colored ones. As time goes by, the soot-colored moths produce more and more similarly colored moths, who are well hidden from the birds AND after enough time, the only moths that remain are soot-colored. This is why so many species “fit” exquisitely into their environment. They have ALL adapted in some way similar to the soot-colored moth.
Answer:
Circulatory system
Explanation:
From the right atrium through the tricuspid valve to the right ventricle through the pulmonary sigmoid valve to the pulmonary trunk to the right and left lungs to the capillary beds of the pulmonary veins to the left atrium to the left ventricle of the heart through the mitral valve, to the aorta through the aortic semilunar valve, to the whole body, to the systemic arteries, to the capillaries of the body tissues, to the systemic veins, to the superior cava vein and inferior cava vein, which enter the right atrium of the heart.
