Included in this infraclass are dogs, cats, bears, whales, monkeys, humans, etc. The term is also frequently used to refer to the appearance of a new species. More specifically, it is change in the gene pool frequencies of a population from one generation to the next. In the Linnaean classification system, genus is the category immediately above species . Pregnancy is another word for gestation.
Mammals are heterodonts.
Lower down are levels of less inclusive sub-categories. The lowest sub-category is a species. Homologies are also referred to as homologous structures. Homoplasies are also referred to as homoplastic structures . All animals other than fish, amphibians, reptiles, birds, and mammals are invertebrates.