What idea of darwin's based on fossils and the modern organisms he found basically says that genetic changes are inherited by later generations?

Basically the evolution of species which was his main theory implies that species evolve from usually lower to higher forms and for example mankind or homo sapiens had ancestors that developed upright posture as opposed to the apes and this upright posture was preserved throughout the various human-like species like Neanderthal Man, Peking Man etc. right up to the present time. 

The answer is observations and interpreting.