The narrator gives some large-scale views on what the Martian invasion taught us. (For one thing, it has taught us to finish our essays early, because if the Martians invade, we might very well be interrupted.)
Lesson #1: The Martian invasion has taught us a lot about science, even though we haven't yet cracked the mystery of the Heat-Ray and the Black Smoke. Although dogs and birds ate many of the dead Martians, there is at least one preserved specimen. From that one body, we learn a lot about Martians.
Lesson #2: But besides learning about science and the Martians, the biggest thing we figured out is that Earth is not alone. Thanks to the invasion, we learned that Earth is not a "fenced in and secure" place for people – we always have to be on our guard for the unexpected. Which is a good thing because now we won't be so darn decadent.
Lesson #3: The Martian invasion helped emphasize how much people have in common. For instance, all of us humans have blood and we like to keep it on the inside – that's something that's pretty common to people.
Although the narrator also notes that there's some evidence that the Martians have landed on Venus, so maybe they won't invade us again. The narrator also notes that if the Martians can go to Venus, then there's no reason we shouldn't do the same when the sun cools and the Earth becomes uninhabitable. That way, humans could survive the death of the Earth. Yay humans. (Or, if you're on Venus: cue threatening music.)
However, the narrators says it's possible that Earth is doomed and the Martians are going to survive. Either option seems fine to him. Why? Let's discuss this strange ending in "What's Up with the Ending?"
The narrator feels a sense of doubt about the invasion and its happy conclusions. When he looks around, he sees the Martian invasion again, and instead of people on the streets, he sees dead bodies, "tattered and dog-bitten" (2.10.11).
Lastly: "And strangest of all is it to hold my wife's hand again, and to think that I have counted her, and that she has counted me, among the dead" (2.10.13). To which we have to respond, "Yes, how strange that that's the last line of this book."