CLASSIFICA TION 27 



of the skate may be given as a possible example of a rudi- 

 mentary organ. Secondly, an organ may change its function 

 or, in other words, may lose its primary function but be 

 preserved and greatly modified by acquiring another function. 

 The skin-armour of placoid scales in sharks is not found as 

 such in higher vertebrates, except the few in the neighbour- 

 hood of the jaws, which form teeth. Again, the appendages 

 of the crayfish show every step in modification frpm the 

 primitive biramous swimming organ to the leg, jaw, or feeler, 

 in accordance with the various functions they have acquired. 



Classification. — Hence we have seen that the animal 

 kingdom forms an ascending series of organisms of struc- 

 tural complexity, which is due to three kinds of gradations. 

 Firstly, animals show a gradation in symmetry from the 

 simple centro-symmetry to the complex piano-symmetry. 

 Secondly, they show a gradation in construction from simple 

 cells to many-layered individuals. Thirdly, they show a 

 gradation in structure due to the functional division of 

 labour. If these gradations were absolute we could form 

 no classification. It would be impossible to divide the 

 animal kingdom into groups if it presented a continuous 

 gradation in structural characters. The breaks in structural 

 sequence permit us to define certain animals and to separ- 

 ate them from certain others. 



Whilst our classification is based primarily upon structural characters 

 there is an important reservation. We have seen in the introduction 

 that structural similarity is called homology and that there are two kinds 

 of homology, inherited and acquired. The acquired homology is often 

 very difficult to distinguish from the inherited homology, but the ideal 

 classification to which all zoologists aspire is based purely upon inherited 

 homology or upon homogenetic characters ; if we place together in one 

 group a number of individuals because they have honioi^enous similarity 

 in structure, we shall by our definition be correlating animals which 

 are descended from a common stock. This is a natural classification, 

 for in it we strive to give expression to the natural relationships of the 

 animals. Let us take a very simple example. If we decide to put 

 in one group the animals which swim in the sea, have a tail-fin and 

 pectoral fins and are of a fish-like shape, we create a group containing 

 the whales and fishes. This is an artificial classification, for further 

 examination shows that the whale agrees with land-mammals in nearly 

 all the most important mammalian characters and that its fish-like shape 

 is acquired or due to adaptation to an aquatic life. 



The determination of natural afifinities is largely helped by the study 

 of embryology and of palceontology, but there is no exact criterion for 



