THE EVOLUTION OF ANIMALS. 385 



All this is entirely without explanation if animals 

 have not been subject to . gradual evolution through 

 the ages. 



383. Evidences from General Similarities of Structure. 



In studying animals we need to keep in mind not merely 

 the varieties and dissimilarities which we see arising, 

 but the underlying likenesses as well. This underlying 

 likeness of structure is really an evidence of relationship ; 

 which is another way of saying that animals, in spite of 

 their differences, are apparently descended from a com- 

 mon stock. If, for example, we examine the offspring 

 of a given pair of parents, as a litter of kittens or of pup- 

 pies, we expect these individuals to be more alike in be- 

 havior, in disposition, and structure than will be the same 

 number of offspring of totally different parents. This 

 likeness is a sign and measure of their kinship. Two 

 spaniels or two newfoundlands are more like each other 

 than either is like to any other breed of dogs. The 

 greater similarity is again the sign of their greater kinship. 

 But the spaniel is more like the newfoundland than 

 either is like the wolf, for the spaniel and the newfound- 

 land probably belong to one species (Canis familiaris] 

 and the wolf to another (Canis lupus) . 



The wolf and dog are more alike, and hence closer akin, 

 than either is to the members of the cat family. The 

 dogs and wolf belong to the genus Canis and the cats to 

 the genus Felis. This is the way we express their de- 

 grees of difference. But in turn the dogs and cats are 

 much more similar to each other than they are to the 

 horse and cow. Because of this similarity in structure 

 and behavior, the dogs and cats are classed together as 

 carnivores, and the horse and cow together as ungulates. 

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