Summary
This book (as the title implies) is interested in the emergence of species, and now that we are talking about it, what are species to begin with? At the time, there were many very experienced naturalists who would disagree on whether a certain plant or animal belonged in species A as a variety or actually should be considered a distinct species. These disagreements arose due to the fact that no one had a clear theory as to how these differences arose and up to then we used internal and external structure to define species. If these structures diverged enough, we would separate them into species. They also used the concept of whether entity A could potentially breed with entity B. If not, then the two entities must not be in the same species. There were other methods used, but these two should suffice to show how difficulties in definition might arise. For example, the internal and external structure of sexed organisms may very greatly as in humans, but we obviously must be the same species, right?….. Actually, that might explain some things. As for mating, this brings its own difficulties (I’m on a roll), but strictly speaking, you can have two different species reproduce, like a grizzly bear and polar bear, and this happens quite often in the plant kingdom. On the other hand, you have many instances of creatures in the same species category that can’t reproduce inside their own species. Think Mastiff and Chihuahua. So these principles are by no means black and white. The prevalent view at the time was the belief that each species was created individually, with this view the categorization of species should be much more straight-forward than it is, but Darwin had other ideas. Using pigeons, due to their availability and the long history they have of human selection, he built an argument. He stated that if we had found the various varieties of pigeons in the wild, we would without a doubt classify them as different species. We only refrain from doing so because we happen to know in this case that all the varieties descended from rock pigeons. What if this same concept applied to the whole animal kingdom? What if instead of individually created species there was a single progenitor which has given rise to the magnificent variety we observe today? How far might these accumulated minute changes take us?
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