Positive evidence is the information available to a child about which groups of words are correct grammatical sentences of a particular language. Basically, a four year old child only produces or hears positive examples of UG from others such as parents or other carers. Infact, studies carryed out by Pinker, Stromswold, Crain and Marcus et al, found that for every rule that was observed, children of three obeyed them the majority of the time. Acquisition of UG may be thought to be innate but it is unable to strive alone. Only in cooperation with environmental cues will it succeed.
Most children have access to spoken language.
They need some form of linguistic input to acquire a language. For one, there are exceptions to every rule. In the case of language, there are no doubt more than one. Deviations of these kind probably answer up to the environmental exposure everyone faces. Subsequently, language develops both with the innate UG device and the environmental exposure of the language.