In Darwin's time there was no agreed-upon model of heredity; in Chapter I Darwin admitted, "The laws governing inheritance are quite unknown." He accepted a version of the inheritance of acquired characteristics (which after Darwin's death came to be called Lamarckism), and Chapter V discusses what he called the effects of use and disuse; he wrote that he thought "there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited", and that this also applied in nature. Darwin stated that some changes that were commonly attributed to use and disuse, such as the loss of functional wings in some island dwelling insects, might be produced by natural selection. In later editions of Origin, Darwin expanded the role attributed to the inheritance of acquired characteristics. Darwin also admitted ignorance of the source of inheritable variations, but speculated they might be produced by environmental factors. However, one thing was clear: whatever the exact nature and causes of new variations, Darwin knew from observation and experiment that breeders were able to select such variations and produce huge differences in many generations of selection. The observation that selection works in domestic animals is not destroyed by lack of understanding of the underlying hereditary mechanism.

What is the theory that is based on acquired characteristics within a species?