Note 3 — INTRODUCTION. ETHNIC GROUPS AND ZOOLOGICAL SPECIES.
Reading edition
The most recent definitions of species given by Wallace and Romanes approximate closely to that of Cuvier. Eimer has suggested another, based solely on the physiological criterion. His definition has the advantage of covering cases of polymorphism, in which the female gives birth to two or several individuals so unlike that we should not hesitate to classify them in two species if guided only by morphology.