"This puts the taxonomist in a quandary. Shall he recognize species which he cannot diagnose? An author who adheres to a strictly morphological species definition may and will refuse to recognize sibling species that are morphologically indistinguishable. In doing so, he ignores the fact that species are not his own artificial creations, but products of nature! A taxonomist who thinks in biological terms will accept these sibling species more readily." -- p. 207This work is quoted and referenced so frequently in the evolution literature that I was quite pleased to see a new edition appear in 1999 that I could sink my teeth into. Amazingly, Ernst Mayr writes a 23 page introduction to the new version even at the ripe old age of 95. How many other authors have had the chance to write a new introduction to a book they wrote more than 55 years in the past?
The text of the original chapters has remained unchanged in this re-publication. Mayr highlights the important parts of each chapter in the introduction and alludes to which areas he was proven correct or incorrect on. For the most part, Mayr was right on, or not far from, the mark. Systematics and the Origin of Species from the Viewpoint of a Zoologist reads like a very fresh work despite the vast accumulation of evidence since its original publication. The introduction is--shall we say--less than humble. Mayr considers himself "ahead of my time" and flaunts how others "followed my lead" (p. xxxiii). He is correct, and the rest of the book isn't nearly as boastful.
The first several chapters are an outstanding introduction to taxonomy (otherwise called "systematics" in some countries other than the United States). Mayr tracks the history of taxonomy from before Linnaeus, to his then current day, to changes he perceives will happen in the future for the science to become more objective. The major problem with the evolution of taxonomy is that it was created before Darwin and was too slow in adopting the evolutionary relationship between species. Physical characteristics (morphology) were the primary factor in distinguishing species, genus, etc. As Mayr points out, however, there is frequently more morphological variability within a single species than there is between different species. A better method to species determination requires understanding how and when species split as one of its elements.
The theory of evolution solved the puzzle of the high degree of perfection of the natural system in a manner that was as simple as it was satisfactory: The organisms of a 'natural' systematic category agree with one another in so many characteristics because they are descendants of one common ancestor! The natural system became a 'phylogenetic' system. The natural system is based on similarity, the phylogenetic system on the degree of relationship. It seems probable that a complete change of the classification is necessitated by changing the criterium on which the system is based. (p. 276)And this is the crux of Mayr's book. Darwin and Linnaeus needed to become more integrated (in the minds of pre-20th century taxonomists at least) in order to classify living things (or previously living things) according to descent--rather than just relying on morphology.
Systematics and the Origin of Species from the Viewpoint of a Zoologist is loaded with examples for dozens of areas in evolutionary biology. If you are looking for documentation for things such as macroevolution, speciation, geographic variation, polytypic species, ring species, hybrids, reduced fertility between geographic races of one species, allochronic species (and their non-existance with a complete fossil record) and other related topics then this is an excellent place to start. For those wanting to understand the process of evolution and see it in "real time" (mostly through the countless examples--including numerous tables quantifying the effects geographical isolation are having on populations that descended from a common stock), I highly recommend this book.
"...every one of the lower systematic categories grades without a break into the next on: the local population into the subspecies, the subspecies into the monotypic species into the superspecies, the superspecies into the species group." -- p. 172from the publisher:
Ernst Mayr is Alexander Agassiz Professor of Zoology, Emeritus, at Harvard University. He is also the recipient of numerous honorary degrees and awards, including the Crafoord Prize for Biology, the National Medal of Science, the Balzan Prize, and the Japan Prize.