This book caught a lot of flack in its day from various critics. One of their chief complaints was, "How can all these animals and environments from so many geographically diverse areas of the world come together on one tiny island?" and the like. Thoughtful people who read this book also tend to notice, and become incredulous.
They are perfectly right, of course; the Mysterious Island cannot possibly exist! Jules Verne might as well have placed penguins and polar bears there to mingle and frolic happily. He was fully aware of this; it was intentional.
The Island is a symbol for the Earth, and the settlers a symbol for Humanity. Verne was first and foremost one thing: a believer in Science, Invention and Technology. This is a novel about human ingenuity conquering the world. Much as F. Scott Fitzgerald represents the Roaring Twenties to a T, Verne represents the spirit of 19th century European technological civilization, just coming into its own. It is no accident that almost all of his main novels were intimately connected with invention: 20,000 Leagues Under the Sea, From the Earth to the Moon, Journey to the Center of the Earth, etc. ad nauseam.
That having been said, I enjoy this novel for its very specific and exacting descriptions of each of the settlers' innovations. They encounter a challenge, they fix it using the remaining implements of civilization and the vast resources of the island. It makes me feel like the world really can be conquered with tame onagers and a few sheets of zinc.