The difficulties that one would face in classification of animals if common fundamental features are not taken into account are as following:

(i) Animal kingdom consists of a large number of organisms. If the common fundamental characteristics are not considered for classification, each organism will have to be placed in a different group. Thus, the study of the organisms would become nearly impossible and it would be very tough.

(ii) Without common fundamental features, it would be very difficult to segregate the organisms into groups.

(iii) Without segregation into groups, comparing different organisms and judging their individual evolutionary significance would be difficult. We would not be able to deduce the evolutionary relationship.