EVIDENCES FROM CLASSIFICATION 97 



importance. These resemblances, though so intimately connected 

 with the whole life of the being, are ranked as merely "adaptive or 

 analogical characters": but to the consideration of these resemblances 

 we shall recur. It may even be given, as a general rule, that the less 

 any part of the organisation is concerned with special habits, the more 

 important it becomes for classification. As an instance: Owen, in 

 speaking of the dugong, says, "The generative organs, being those 

 which are most remotely related to the habits and food of an animal, 

 I have always regarded as affording very clear indications of its true 

 affinities. We are least likely in the modifications of these organs to 

 mistake a merely adaptive for an essential character." With plants 

 how remarkable it is that the organs of vegetation, on which their 

 nutrition and life depend, are of little signification; whereas the 

 organs of reproduction, with their product the seed and embryo, are 

 of paramount importance! So again in formerly discussing certain 

 morphological characters which are not functionally important, we 

 have seen that they are often of the highest service in classification. 

 This depends on their constancy throughout many allied groups; and 

 their constancy chiefly depends on any slight deviations not having 

 been preserved and accumulated by natural selection, which acts only 

 on serviceable characters. 



WHAT IS A SPECIES? 



"Each kind of animal or plant, that is, each set of forms which 

 in the changes of the ages has diverged tangibly from its neighbors, 

 is called a species. There is no absolute definition for the word 

 species. The word kind represents it exactly in common language, 

 and is just as susceptible to exact definition. The scientific idea of 

 species does not differ materially from the popular notion. A kind of 

 tree or bird or squirrel is a species. Those individuals which agree 

 very closely in structure and function belong to the same species. 

 There is no absolute test, other than the common judgment of men 

 competent to decide. Naturalists recognize certain formal rules as 

 assisting in such a decision. A series of fully intergrading forms, 

 however varied at the extremes, is usually regarded as forming a single 

 species. There are certain recognized effects of climate, of climatic 

 isolation, and of the isolation of domestication. These do not usually 

 make it necessary to regard as distinct species the extreme forms of 

 a series concerned." 1 



' From D. S. Jordan and V. L. Kellocg, Evolution and Animal Life. 



