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the relatiouship of ancestor and descendant, are treated as members of different 

 families ; if it is right to keep ancestor and descendant, in spite of the iutergradations 

 which have existed according to the theory of evolution, in separate families in this 

 case, why do we not do it in the case of the horses which our forefathers rode and 

 those which live now ? Tf it is the presence of morphological ditferonce which leads 

 us to split uj) in the one case, and the absence of such difference in the otlier case to 

 unite, why then are Distomum, Media, and Cercaria ; Rhahdonema and Rhabditia ; 

 Vanessa levana and its offspring prorsa, the same species ? IMorphological difference 

 alone is not a criterion of siiecific distinctness. 



Besides the morphological differences among animals we observe a mental or 

 psychological difference. It has often been noticed that in mixed flocks of sheep • 

 or cattle the individuals belonging to the same race often keep separate from the rest, 

 and it is also well known that in the state of nature strangers are driven away or 

 even killed. Though psychological selection, as manifested in these cases, is of 

 importance to accelerate the transformation of a variety and to fix the varietal 

 characters, we believe that among domesticated animals it is very often not racial 

 community that keeps the individuals together, but the circumstance of being 

 accustomed to one another. In some districts of Germany all the geese of a village 

 are driven to pasture in one large flock ; when on the pasture ground, the individuals 

 belonging to each house keep together, even if they are not the offspring of the same 

 parents, but are brought together from different villages as goslings : this keeping 

 together is an expression not of community of characters, nor of community of descent, 

 but of community of the stable. 



For us systematists that kind of psychological variation is of more interest which 

 is the immediate outcome of morphological differences in the organs of sense and in 

 the organs which are destined to affect the senses. The variability of the organs of 

 sense among higher animals, and the difference of discriminating power among the 

 individuals of the same species and race, are facts so weU known that it is sufficient to 

 remind the reader of the variability in the eyesight of men, or of the difference between 

 dogs in regard to the ])ower of smell. Among lower animals the senses are often very 

 differenth' developed in the various families; in insects the power of discriminating 

 form and colour seems generally to be rather weak, while in some families the organ 

 of smell is highly developed. Carrion- and dung-beetles are able not only to smell the 

 carrion or dung from a great distance, but, what is more important, they distinguish 

 between the scent of carrion and that of dung. The sexes of Lepidoplera and 

 Coleoptera are brought together by differences in the scent of the sexual scent-glands 

 of the various forms ; females of certain moths attract great numbers of males, even 

 if the /ema/e is kept in a box with holes, and there is not the least doubt that the 

 males of the various species of a genus follow the scent of the respective females and 

 not that of allied species. As individuals of one species sometimes come to a femaU 

 of another species, and as, further, dung-beetles are occasionally attracted by carrion, 

 wo must conclude that the specimens of dung-beetles really perceive the scent of 

 dung and carrion, and that the males of insects not only perceive the scent of the 

 /errt«?es of their own species, but also that of other species. The phenomenon that 

 dung-beetles come to dung, and that the males of a given species of insect follow the 

 scent oi the females of their species, cannot be explained by the assumption that the 

 organ of smell of the specimens is so constructed that the insect is not able to per- 

 ceive any scent but that particular one. The specimens of allied species, therefore, 

 * D.-invin, Variation nf Animah, etc.. 2nri Kil. London, 1888. p. Uo. 



