It was about 1200 B.C.that the primitive Dorian tribes moved south-ward into the Greek peninsula,leaving their northern mountain fastnesses for more favorable territory,and subsequently their chief tribe,the Spartans,developed the city of Sparta.A large section of the former inhabitants of the invaded region fled, for self-preservation, to Asia Minor and the Ionian islands of the Aegean Sea, where in time they established Greek trading colonies.It was in these colonies, in the sixth century B.C., that the Ionian school was founded and Greek philosophy blossomed and demonstrative geometry was born.