176 OUTLINES OF CLASSIFICATION. 



classification, and in this system every plant and animal has its 

 fixed place, with definite relationships to every other plant and 

 animal. It is equally plain, however, that the natural system of 

 classification cannot be fully laid down until the ancestral history 

 of plants and animals has been completely ascertained. This 

 task has not yet been accomplished, and any attempt to formulate 

 the natural system must therefore be at present only approxi- 

 mate and tentative. It is natural that authorities should differ 

 more or less widely in these attempts, and hence a number of 

 systems are now in vogue, agreeing in their main outlines, but 

 differing considerably in details. 



Correlation of Structure. The labor of determining the natu- 

 ral classification is much lightened by the fact that certain struc- 

 tures are often found as a matter of experience to be constantly 

 associated or correlated, so that the presence of one indicates the 

 presence of the others. In such cases a single character may be 

 taken as the basis of a classification which is natural, because 

 agreement in the one character has been proved empirically to 

 indicate agreement in many others. For example, it has been 

 proved that the differences or resemblances in the structure of 

 mammals are almost invariably correlated with corresponding 

 differences or resemblances in their teeth. Hence mammals, to 

 a great extent, can be naturally classified according to the struc 

 ture and disposition of the teeth. And so in any group it is 

 usually possible to discover empirically some one or few charac- 

 ters on which, by reason of their constant association with other 

 characters, a natural classification can be based. 



Homology and Analogy. A structural similarity due to com- 

 mon descent is known as Homology. Examples of mutually 

 homologous organs are the wing of a bird, the fore-leg of a dog, 

 the flipper of a seal, and the fore-limb of a mole. Although 

 these organs perform utterly different functions they exhibit a 

 fundamental likeness or homology in structure, due to the fact 

 that these different forms of fore-limb are descended from the 

 fore-limb of a common ancestor, their divergence in function 

 being a matter of secondary importance. 



On the other hand, a similarity in function, whether accom- 

 panied or not by a similarity of structure, is known as Analogy. 

 The wing of a bird and the wing of a bat are not only homolo- 

 gous in structure, but also analogous in function. The wing of 



