CHAPTER XXI. 
ANIMAL RELATIONSHIPS. 
Classification. In the earlier days of zodlogical 
“research, animals were classified by their resemblances 
and differences as shown by adult forms. As informa- 
tion concerning animal structure increased it became 
more and more difficult to arrange schemes of classifi- 
cation. The well-known classification of Cuvier divided 
the whole animal kingdom into four great groups: 
Radiates, Mollusks, Articulates, and Vertebrates. 
This classification assumed that species are changeless, 
and the subject of kinship was not considered of so 
great importance as it is now. 
Because of the increasing study of relationships, 
these groups were gradually abandoned for new ones. 
Radiates became Protozoa, Porifera, and Ccelenterata; 
and Articulates became Arthropoda and Vermes. 
Many other changes have been made, and _ similar 
changes are constantly being made, because relation- 
ship by descent is now thought to be of so great 
importance. Indeed, relationship is nowadays con- 
sidered to be of far more consequence than classifica- 
tion. , 
Structure. Resemblance in structure must always 
be the most important guide in discovering relation- 
ship, as it has always been the basis of classification. 
But such resemblances must be more than superficial, 
and they must be sought for in other than the common 
adult forms of animals. 
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