But these sense-perceptions have been all the time spurring the mind to begin a higher work.  At first it is conscious merely of objects, and its main effort is to gain a clearer and clearer perception of these.

Now it is led to undertake, so to speak, the work of a sense-organ of a higher grade.  It begins to directly see invisible relations just as truly as through the eye it has perceived light.  First perhaps it perceives that certain perceptions and experiences, agreeable or disagreeable, occur in a certain sequence.  It begins to associate these.  It learns thus to recognize the premonitory symptoms of nature’s favor or disfavor, and thus gains food or avoids dangers.  The bee learns to associate accessible nectar with a certain spot on the flower marked by bright dots or lines, “honey-guides,” and the chimpanzee that when a hen cackles there is an egg in the nest.  But association is only the first lesson; inference and understanding follow.

The child at kindergarten receives a few blocks.  It admires and plays with them.  Then it is taught to notice their form.  After a time it arranges them in groups and learns the first elements of number.  But when it has advanced to higher mathematics, the blocks, or figures on the blackboard, become only symbols or means of illustrating the great theorems and propositions of that science.  Thus the animal has begun in the kindergarten way to dimly perceive that there are real, though intangible and invisible, relations between objects.  But what is all human science but the clearer vision, and farther search into, and tracing of these same relations?  And what is all advance of knowledge but a perception of ever subtler relations?  What is even the knowledge of right but the perception of the subtlest and deepest and widest relations of man to his environment?  The animal seems to be steadily advancing along the path toward the perception of abstract truth, though man alone really attains it.

And the higher power of association and inference which we call understanding, aided by memory, results in the power of learning by experience, so characteristic of higher vertebrates.  The hunted bird or mammal very quickly becomes wary.  A new trap catches more than a better old one until the animals have learned to understand it, and young animals are trapped more easily than old.  Cases showing the limitations of mammalian intelligence are interesting in this connection.  A cat which wished to look out and find the cause of a noise outside, when all the windows were closed by wooden blinds, jumped upon a stand and looked into a mirror.  Her inference as to the general use of glass was correct; all its uses had not yet come within the range of her experience.  A monkey used to stop a hole in the side of a cage with straw.  The keeper, to tease him, used to pull this out.  But one day the monkey tugged at a nail in the side of his cage until he had pulled it out, and thrust it into the hole.  But when