480 ZOOLOGY 



484. 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 under- 

 lying 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 common stock. If, for example, we examine 

 the offspring of a given pair of parents, as a litter of kittens or of 

 puppies, we expect these individuals to be more alike in behavior, 

 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 newfound- 

 lands 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 newfoundland 

 probably belong to one species (Canis jamilaris) 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 degrees 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. 



With all their differences the carnivores and the ungulates 

 have many fundamental points in common. They are all warm- 

 blooded, covered with hair, have mammary glands, carry the 

 young in the uterus attached by a placenta. Hence they belong 

 to the subclass placentals and to the class of mammals. They 

 have similar parts to their skeletons, similar arrangements of 

 the principal muscles, similar structure of the brain and central 

 nervous system. Thus it might be shown that the newfound- 

 land and the spaniel are similar to all the vertebrates, and 

 finally to all the animals. 



If the similarities of structure in a litter of kittens is a sign 

 of kinship, we may equally believe that the similarities between 



