The first and sixth sentences are given in the beginning a s S1 and S6. The middle four sentences in each have been jumbled up and labelled as P, Q , R , and S . You are required t o find the proper sequence o f these four sentences
S1 : Every geological period has been dominated by one particular form of life; as conditions changed to those of a new period, different types of organisms emerged into dominance.
S6 : The origin of completely new biological systems is comparatively rare, so consequently only a few major types have developed during the history of life.
P : Individual shifts rely on opportunity, ecological access, and the development of a new adaptation.
Q : Characteristically these shifts are rapid and are followed by a bout of adaptive radiation into the new ecological zone — for example, with the emergence of life on land.
R : Within each period, natural selection has created a vast array of adaptive experiments culminating in a breakthrough to a new biological system that may cross the ecological barrier to the next period.
S : Every breakthrough of shift in dominance is dependent on ones that have occurred before so that the process of evolution is progressive.