Because of some reasons we can say that XML Schema are successors of DTD.Some reasons are given below:

1. As DTD, XML Schemas also written in XML.
2. As DTD, XML Schemas also support Data types and Namespaces.
But, XML Schemas is much good as DTD. Because it is much richer and powerful than DTD. And it also has some features that are made to solve problems comes in future.