Page:The Science of History and the Hope of Mankind.djvu/26



Now, all those forces and materials that constitute the endronment, e,g., light, heat, air, water, soils, food substances, etc., are not equally necessary to the life and development of every organism; in fact, some are positively harmful and injurious to its interests. Besides, among the living beings themselves there are relations of mutual alliance and rivalry. It is the interaction and resultant of all the forces of Nature, both favourable and unfavourable to life, that determine the development and growth of every individual organism. And so the form and characteristics of every living being depend on the nature and strength of these contending forces.

Thus, in the vegetable and animal worlds the varieties of form and colour, structural and external characteristics,