GENERAL BIOLOaY. 35 



any classification must be regarded but as the temporary creed 

 of science, to be modified with, the extension of knowledge. As 

 a matter of fact this has been the history of all zoological and 

 other systems of arrangement. The only purpose of grouping 

 is to simplify and extend knowledge ; this being the case, it fol- 

 lows that a method of grouping that accomplishes this has 

 Yalue, though the system may be artificial that is based on 

 resemblances which, though real and constant, are associated 

 with differences so numerous and radical that the total amount 

 of likeness between objects thus grouped is often less than the 

 difference. Such a system was that of Linnaeus, who classified 

 plants according to the number of stamens, etc., they bore. 



Seeing that animals which resemble each other are of com- 

 mon descent from some earlier form, to establish the line of de- 

 scent is to determine in great part the classification. Much as- 

 sistance in this direction is derived from embryology, or the 

 history of the development of the individual (ontogeny) ; so 

 that it may be said that the ontogeny indiciates, though it does 

 not actually determine, the line of descent (phylogeny) ; and 

 it is owing to the importance of this truth that naturalists have 

 in recent years given so much attention to comparative embry- 

 ology. 



It will be inferred that a natural system of classification must 

 be based both on function and structure, though chiefly on the 

 latter, since organs of very different origin may have a similar 

 function ; or, to express this otherwise, homologous structures 

 may not be analogous ; and homology gives the better basis for 

 classification. To illustrate, the wing of a bat and a bird are 

 both homologous and analogous ; the wing of a butterfly is 

 analogous but not homologous with these ; manifestly, to clas- 

 sify bats and birds together would be better than to put hirds 

 and insects in the same group, thus leaving other points of re- 

 lationship out of consideration. 



The broadest possible division of the animal kingdom is into 

 groups, including respectively one-celled and many-celled forms 

 — i. e., into Protozoa and Metazoa. As- the wider the grouping 

 the less are differences considered, it follows that the more sub- 

 divided the groups the more complete is the information con- 

 veyed ; thus, to say that a dog is a metazoan is to convey a cer- 

 tain amount of information ; that he is a vertebrate, more ; that 

 he is a mammal, a good deal more, because each of the latter 

 terms includes the former. 



