In darwin's day, most people believed that the characteristics of offspring are an average of their parent's characteristics. this concept is called inheritance.

Before the postulates of modern genetics were written by Gregor Mendel, the popular model for explanation of inheritance was the blending inheritance.
It was believed that the two parent's characteristics blend when they produce offspring so that the offspring have blended traits of their parents.
This was one of the biggest problems Darwin has encountered while working on his evolutionary theory. If every new generation was a blend of their parents then all variation would be lost, since the offspring would always have blended characteristics. For example, if we crossed two flowers, one that is red and other that is yellow, in the next generation we would have orange flowers.
Only after Gregor Mendel realized while doing his experiments on peas that the traits, some being dominant in expression and some being recessive are inherited separately the concept of blended inheritance could be disproved.