I believe the answer is:
the human race is a distinct species and is not confounded with another
the quadrupeds are a distinct d=family
the other families are more or less distinct in this process
The whole experiment was conducted with the purpose to know the basic human nature of humans and compared it to the nature of other animals in the zoo. Twain concluded that regardless of the difference in intelligence, animals tend to act more civil with one another compared to humans.