Platypus is a popular account of the discovery of the Platypus by European science in 1798 (it was of course well-known to the aboriginal population) and the controversies it caused among scientists. Moyal explains how biologists were confused by the combination of mammalian traits such as fur and a four-chambered heart with a bill that they thought was birdlike (actually this was an artifact due to the hardening of the "bill" in prepared specimens; the actual organ in the living animal is very flexible and cartiliginous) and a reproductive apparatus similar to reptiles (the uterus, bladder and anus end in a common orifice, the cloaca, which accounts for the term "monotreme" meaning "single orifice" now used for the order Monotremata). At first they were unsure where it fit in the "chain of being", whether to consider it as a reptile, bird or mammal, or as a separate class of its own (together with the echidna, which has a similar combination of features.) Later, there was a controversy when the mammary glands were discovered over whether they were actually milk-producing for suckling the young or for some other purpose, since there is no actual teat or nipple. There was also controversy over whether it was viviparous (bringing forth live young) like mammals, oviparous (egg-laying) like most reptiles and all birds, or oviviviparous (hatching the egg internally) like some reptiles.
Much of the problem in solving these questions, according to Moyal (who is Australian) was the contempt of British and European scientists for the amateur Australian naturalists who were actually producing factual descriptions, but were diffident about theory, leaving that to the mother country (not to mention for the aboriginal population who knew the truth that the platypus laid eggs but were considered "unreliable" witnesses.)
There is much about the role of Richard Owens, who became the expert on the monotremata and marsupials in the middle of the nineteenth century; Moyal describes him as an expert on reconstructing extinct species from fragmentary fossils (he was called the British Cuvier) but out of his depth in theory, and a rigid Anglican who was part of the establishment. She describes him in his later life as arrogant, and as the most hated scientist in England, with a habit of taking credit for other researchers' discoveries. This is corroborated by the descriptions I have read of him in other books on the history of biology; I have a biography of him on my TBR list, which I hope to get to next year after I retire. Owens denied as long as possible that the mammary glands were real and argued for oviviviparity. (Many of those who considered it a mammal insisted it must be viviparous, but Owen knew that could not be the case given the primitiveness of the uterus; but he could not accept that it was actually oviparous, which was mainly the view of those who considered it a reptile or bird.)
Moyal also shows the role the platypus played in Darwin's thinking about evolution and the debates over the theory. Darwin described Owen as the one opponent he actively disliked. Lyell, whom Darwin respected highly and was particularly concerned to convince, initially opposed his theory as "progressive", i.e. as claiming that there was a progression from lower to higher forms, as was the case with Lamarck and other previous evolutionists, and used the platypus as a counterexample; Darwin wrote back that his theory did not assume a necessary "progress" from less to more complex, but only that species became more fit for their environments, which could involve becoming less as well as more complex.
Nevertheless, after professional (and Darwinian) scientists proved by field studies that the platypus in fact laid eggs, the monotremata were considered as a link in the development from reptiles through marsupials to placental mammals (correct) which had simply not evolved further (not correct). In the 1980s it was discovered that the platypus had a system of electrolocation unique in the animal kingdom (some fishes have electric organs but they work in an entirely different way) which shows that the platypus had a long evolution, which simply went in a different direction than other mammals.
I was expecting to read that like so many unique animals the platypus was endangered, but that is apparently not the case; they still thrive in their native habitat and are a protected species throughout, although pollution of rivers creates a certain danger.