But if there was one sin above another which called for the destruction of the race by the flood, it was the base crime of amalgamation of man and beast which defaced the image of God, and caused confusion everywhere. God purposed to destroy by a flood that powerful, long-lived race that had corrupted their ways before him. He would not suffer them to live out the days of their natural life, which would be hundreds of years. It was only a few generations back when Adam had access to that tree which was to prolong life. After his disobedience he was not suffered to eat of the tree of life and perpetuate a life of sin. In order for man to possess an endless life he must continue to eat of the fruit of the tree of life. Deprived of that tree, his life would gradually wear out. {1SP 69.1}

More than one hundred years before the flood the Lord sent an angel to faithful Noah to make known to him that he would no longer have mercy upon the corrupt race. But he would not have them ignorant of his design. He would instruct Noah, and make him a faithful preacher to warn the world of its coming destruction, that the inhabitants of the earth might be left without excuse. Noah was to preach to the people, and also to prepare an ark as God should direct him for the saving of himself and family. He was not only to preach, but his example in building the ark was to convince all that he believed what he preached. {1SP 69.2}

Every species of animals which God had created was preserved in the ark. The confused species which God did not create, which were the result of amalgamation, were destroyed by the flood. Since the flood, there has been amalgamation of man and beast, as may be seen in the almost endless varieties of species of animals, and in certain races of men. {1SP 78.2}

Yeah, some people misread her statements and conclude men and monkeys were amalgamated resulting in Negros. But obviously her statements mean animals were amalgamated with animals and humans were amalgamated with humans resulting in variations within genera and species.

First of all, if amalgamation is what produced the heretofore-unknown species and/or genera, then it follows that beforehand such species were unknown.

True.
Species are being produced almost daily. Some by paper only, but there are changes being produced. Ask any scientist, whether creationist or evolutionist.

However, your assumption that amalgamation of man means species, is faulty on a number of reasons.
All man is one species.
All amalgamations does not mean speciation.