He had his forerunners in Montesquieu, Alexander von Humboldt, Buckle, Ritter, Kohl, Peschel and others; but he first investigated the subject from the modern scientific point of view, constructed his system according to the principles of evolution, and based his conclusions on world-wide inductions, for which his predecessors did not command the data.

To this task he brought thorough training as a naturalist, broad reading and travel, a profound and original intellect, and amazing fertility of thought.

Yet the field which he had chosen was so vast, and its material so complex, that even his big mental grasp could not wholly compass it.

His conclusions, therefore, are not always exhaustive or final.
Moreover, the very fecundity of his ideas often left him no time to test the validity of his principles.

He enunciates one brilliant generalization after another.