I think hummingbirds are really amazing he author states that "Each one visits a thousand flowers day. The can dive at sixty miles an hour. They can fly backward. They can fly more than five hundred miles without pausing to 194). A little wings move so quickly that you cannot even see them. The author tells us though how fragile they are because they live their lives at such a fast pace. In addition, he wants to compare the life of a tortoise with hummingbird. A tortoise can live to be two hundred years old because its heart beats so slowly.
A hummingbirds heart eats so very quickly though that they only live a year or two. Even during that short life they are always close to death because of how much energy they must spend just to fly and find something to eat. The author is so clever to begin the speaking about animal hearts and then speaking about the human heart at the end of the essay. He is using the subject of hearts as a metaphor here. I think Doyle contrasts between the hearts hummingbirds and blue whales is just to add interest to the essay.
The hearts of the two returns may both have four chambers as he mentions and they both are organs which pump blood through their bodies. Would say that the last paragraph is the most imaginative part of the essay. He completely departs from listing the facts about the hearts and his personal observations about human emotions and psychology. He talks about how we might want to protect ourselves from the experience of pain, yet how impossible that is because feelings are so much a part of us that they can never turned off.
In my opinion, I guess the author want to use the picture of humans to reveal is opinion. People are always running around doing something and never seem to rest, relax or spend any time with their friends or family. Maybe they are trying to get rich very quickly in order to accomplish their goals. Other people run around drinking and taking drugs. If we try to live our own lives too intensely or quickly, their life will be shortened too. We will live longer than a hummingbird of course, but we might die of a heart attack or high blood pressure in middle age instead of having a long life.