CHAPTER 2.18. DURATION OF THE HISTORY OF OUR STEM. - The Evolution of Man — Volume 2 - 7 - Ernst Haeckel

CHAPTER 2.17. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT.CHAPTER 2.19. OUR PROTIST ANCESTORS.

CHAPTER 2.18. DURATION OF THE HISTORY OF OUR STEM.