This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Organic World is divisible into two kingdoms, the animal and the vegetable. The power of building up the complex molecules of organic matter from the separate elements, or such simple combinations as carbonic acid, water, and ammonia, is peculiar to vegetables, while intelligence is confined to animals. All vegetables, however, do not possess the building power; it is apparently a property belonging exclusively to green parts: and all animals do not possess intelligence; but, on the contrary, it may be assumed to be entirely absent from the lowest forms, while it appears in obscure and gradual dawnings in others.
In the region of the minute and simple beginnings of life, the animal and vegetable kingdoms touch one another, and there may even be a common territory including beings which have no claim to be classed in one rather than the other. But, growth and reproduction being the highest functions of vegetable life, while intelligence is the highest aim exhibited in the animal series, vegetable and animal forms rapidly diverge as they become complex, so that those of a highly developed description in the one kingdom cease to have any resemblance to those of the other.