The writers quite confused about Seward's "white, male, Western superiority. Evidently he must have missed the early record, of Seward's Civil War records, and his argument discussing Slavery, on March 11 1850, in that U.S. Senate Chamber. The "Higher Law Speech" for California Statehood, against admitting another slave state. His delay in the Alaska Transaction with Russia, was also based on his desire to acquire no possessions, until Dixie was driven into the ground. 
"The right to have a slave implies the right in some one to make the slave; that right must be equal and mutual, and this would resolve society into a state of perpetual war. But if we grant the original equality of the states, and grant also the constitutional recognition as slaves as property, still the argument we are considering fails. Because the states are not parties to the Constitution as states; it is the Constitution of the people of the United States."
http://facweb.furman.edu/~benson/docs/seward.htm