Thus mammals and birds take pride of place as objects of nature research. In his book Ever Since Darwin--Reflections in Natural History, Stephen Jay Gould writes that if we find an extra bump on the tooth of a mammal, we classify it as a new species.
《達爾文大震撼》一書作者古爾德就曾表示，哺乳動物牙齒上多了一點隆起，就被界定為新種。