Classification of Fishes 



371 



or coming into use is the result of our best endeavors to accom- 

 plish this purpose, and represents what approach we have made 

 to this end. It is one of the great corollaries of that theorem of 

 evolution which most naturalists are satisfied has been demon- 

 strated. It is necessarily a morphological classification; that is, 

 one based solely upon considerations of structure or form {nop(f>ii^ 

 form, morphe), and for the following reasons: Every offspring 

 tends to take on precisely the form or structure of its parents, 

 as its natural physical heritage; and the principle involved, or 

 the law of heredity, would, if nothing interfered, keep the de- 

 scendants perfectly true to the physical characters of their 

 progenitors; they would 'breed true' and be exactly alike. 

 But counter influences are incessantly operative, in consequence 

 of constantly varying external conditions of environment; the 

 plasticity of organization of all creatures rendering them more 

 or less susceptible of modifications by such means, they become 

 unlike their ancestors in various ways and to different degrees. 

 On a large scale is thus accomplished, by natural selection and 

 other natural agencies, just what man does in a small way in 

 producing and maintaining different breeds of domestic ani- 

 mals. Obviously, amidst such ceaselessly shifting scenes, de- 

 grees of likeness or unlikeness of physical structure indicate 

 with the greatest exactitude the nearness or remoteness of 

 organisms in kinship. Morphological characters derived from 

 the examination of structure are therefore the surest guides we 

 can have to the blood relationships we desire to establish ; and 

 such relationships are the ' natural affinities ' which all classifi- 

 cation aims to discover and formulate." 



Species as Twigs of a Genealogical Tree. — In another essay 

 Dr. Coues has compared species of animals to " the twigs of a 

 tree separated from the parent stem. We name and arrange 

 them arbitrarily in default of a means of reconstructing the 

 whole tree according to -nature's ramifications." If one had a 

 tree, all in fragments, pieces of twig and stem, some of them 

 lost, some destroyed, and some not yet separated from the mass 

 not yet picked over, and wished to place each part where he 

 could find it, he would be forced to adopt some system of nat- 

 ural classification. In such a scheme he would lay those parts 

 together which grcAV from the same branch. If he were corn- 



