Since this book is about mammals, it is reasonable to ask the question: ‘What is a mammal?’. A mammal is a vertebrate, which means it has a vertebral column or backbone – a facetious answer to the question could be that a mammal is a mammal when it is not a fish, an amphibian, a reptile or a bird. All of these are also vertebrates, you see. Most mammals have hair, wool or fur, but not all; some have spines and others scales, whilst man has little or no such covering. So there is great variety among mammals, which makes it all the more confusing when it comes to definition. However, the correct answer to the question is that mammals are those creatures that suckle their young. This distinguishes them from other animals, including the other vertebrates mentioned.

Reproductively speaking, there are three kinds of mammal. There are those that lay eggs, such as the duck-billed platypus and echidna (what used to be called the spiny anteater), those that give birth to embryos – the marsupials, including kangaroos among others (you can accommodate up to a dozen newly born opossums on a teaspoon) – and those such as ourselves, who give birth to relatively well-developed foetuses after nurturing them for longer periods in the womb (the uterus). Each of these groups is classified sequentially as being prototherian, metatherian or eutherian mammals. Thus, the completion of body form occurs in the uterus of eutherian mammals in contrast to that of a ‘joey’ (baby kangaroo or any other marsupial), for example, whose longest period of development is at the teat, having been born as a young embryo.