This paper first gives a simple statistical counting of how many species of animals have been used as subjects in studies published in the following periodicals ; Journal of Comparative and Physiological Psychology, Journal of Experimental Analysis of Behavior, The Annual of Animal Psychology, Behaviour, Animal Behaviour, and Zeitschrift fur Tierpsychologie. The result indicates that, while only a few selected species are used in psychological papers, relatively greater variety of species are studied in papers which appeared in journals of ethology (the last three mentioned above). Since we believe that the fundamental aim of comparative psychology is to reveal the nature of differences found in many species in the animal kingdom, and not the shared properties common to them, comparison of distinct species must be the primary task. We have proposed Behavior-taxonomy, which is based on the method of numerical taxonomy and distinct from normal phylogeny. However, Behavior-taxonomy is by no means intended to preclude the possibility of its contributing to the phylogeny. It is hoped that the success of Behavior-taxonomy would ultimately yield the overall picture of animals, whose nature is the fundamental object of the psychological inquiary.