Among the lower animals the ignorance of one is the ignorance of all, and the knowledge of one is the knowledge of all, in a sense in which the same is not true among men. Of course some are more stupid than others of the same species, but probably, on the one hand, there are no idiots among them, and, on the other, none is preëminent in wit.
Animals take the first step in knowledge--they perceive things and discriminate between them; but they do not take the second step--combine them, analyze them, and form concepts and judgments.
So that, whether animals know much or little, I think we are safe in saying that what they know in the human way, that is, from a process of reasoning, is very slight.
The animals all have in varying degrees perceptive intelligence. They know what they see, hear, smell, feel, so far as it concerns them to know it. They know their kind, their mates, their enemies, their food, heat from cold, hard from soft, and a thousand other things that it is important that they should know, and they know these things just as we know them, through their perceptive powers.
We may ascribe intelligence to the animals in the same sense in which we ascribe it to a child, as the perception of the differences or of the likenesses and the relations of things--that is, perceptive intelligence, but not reasoning intelligence. When the child begins to "notice things," to know its mother, to fear strangers, to be attracted by certain objects, we say it begins to show intelligence. Development in this direction goes on for a long time before it can form any proper judgment about things or take the step of reason.
If we were to subtract from the sum of the intelligence of an animal that which it owes to nature or inherited knowledge, the amount left, representing its own power of thought, would be very small. Darwin tells of a pike in an aquarium separated by plate-glass from fish which were its proper food, and that the pike, in trying to capture the fish, would often dash with such violence against the glass as to be completely stunned. This the pike did for three months before it learned caution. After the glass was removed, the pike would not attack those particular fishes, but would devour others that were introduced. It did not yet understand the situation, but merely associated the punishment it had received with a particular kind of fish.