Outlines of zoology . and lastly, there are thegeneral resemblances of structure which bind Mammals together incontrast to Birds or Reptiles, though all are included in the series orphylum Vertebrata. It must be understood that the real things are the individual animals,and that a species includes all those individuals who resemble oneanother so closely that we feel we need a specific name applicable tothem all. And as resemblances which seem important to one naturalistmay seem trivial to others, there are often wide differences of opinion asto the number of species which a genus contains. But
Outlines of zoology . and lastly, there are thegeneral resemblances of structure which bind Mammals together incontrast to Birds or Reptiles, though all are included in the series orphylum Vertebrata. It must be understood that the real things are the individual animals,and that a species includes all those individuals who resemble oneanother so closely that we feel we need a specific name applicable tothem all. And as resemblances which seem important to one naturalistmay seem trivial to others, there are often wide differences of opinion asto the number of species which a genus contains. But while no rigid definition can be given of a species, certain•common-sense considerations should be borne in mind :— CLASSIFICA TION. 15 I. No naturalist now believes, as Linnjeus did, in the fixity of species;we believe, on the contrary, that one form has given rise to the same time, the common characteristics on the strength of whichwe deem it warrantable to give a name to a group of individuals, must. Fig. 18.—Diagrammatic expression ot classification in agenealogical tree. B indicates possible position of Balano-glossus, D of Dipnoi, S of Sphenodon or Hatteria. not be markedly fluctuating. The specific characters should exhibita certain degree of constancy from one generation to another. 2. Sometimes a minute character, such as the shape of a tooth or themarking of a scale, is so constantly characteristic of a group of indi-viduals that it may be safely used as the index of more important 16 GENERAL SURVEY OF THE ANIMAL KINGDOM. characters. On the other hand, the distinction between one species andanother^ should always be greater than any difference between themembers of a family (using the word family here to mean the progenyof a pair). For no one would divide mankind into species according tothe colour of eyes or hair, as this might lead to the absurd conclusionthat two brothers belonged to different species. Thus it is often doublyunsatisfactory when a
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
