The dicotyledons include all forms of leaf-bearing forest trees, almost all fruits and flowers, in fact by far the greater part of the vegetable world familiar to man, as coming into immediate relation with it, except in the case of the cultivated plants, which are developments of the monocotyledon grasses.
We see, therefore, in the geological record a confirmation of the evolution over immense periods of time of the more complex and perfect from the simple and primitive.
If we turn to the same geological record to trace the development of animal life, we find it running a parallel course with that of plants. The earliest known fossil, the Eozoon Canadiense, from the Lower Laurentian, is that of the chambered shell of a protista of the class of Rhizopods, whose soft body consists of mere protoplasm which has not yet differentiated into cells. As we ascend the scale of the primordial era, traces of marine life of the lower organisms begin to appear, until in the Silurian they become very abundant, consisting however mainly of mollusca and crustacea, and in the Upper Silurian we find the first traces of fishes.
In the Primary era the Devonian and Permian formations are characterised by a great abundance of fishes, of the antique type, which has no true bony skeleton, but is clothed in an armour of enamelled scales, and whose tail, instead of being bi-lobed or forked, has one lobe only—a type of which the sturgeon and garpike are the nearest surviving representatives. In the Coal formation are found the first remains of land animals in the form of insects and a scorpion, and a few traces of vertebrate amphibious animals and reptiles; while higher up in the Permian are found a few more highly developed reptiles, some of which approximate to the existing crocodile. Still fishes greatly predominate, so that the whole Primary period may be called the age of fishes, as truly as, looking at its flora, it may be called the age of ferns.
In the Secondary period reptiles predominate, and are developed into a great variety of strange and colossal forms. The first birds appear, being obviously developed from some of the forms of flying lizards, and having many reptilian characters. Mammals also put in a first feeble appearance, in the form of small, marsupial, insectivorous creatures.
In the Tertiary period the class of mammals greatly predominates over all other vertebrate animals, and we can see the principal types slowly developing and differentiating into those at present existing. The human type appears plainly in the middle Miocene, in the form of a large anthropoid ape, the Dryopithecus, and undoubted human remains are found in the beginning of the Quaternary, if not, as many distinguished geologists believe, in the Pliocene and even in the Miocene ages.
So far, therefore, there seems to be a complete parallelism between the evolution of animal and vegetable life from the earliest to the latest, and from the simplest to the most complex forms. These facts point strongly to a process of evolution by which the animal and vegetable worlds, starting from a common origin in protoplasm, the lowest and simplest form of living matter, have gradually advanced step by step, along diverging lines, until we have at last arrived at the sharp antithesis of the ox and the oak tree. It is clear, however, that this evolution has gone on under what I have called the generalised law of polarity, by which contrasts are produced of apparently opposite and antagonistic qualities, which however are indispensable for each other’s existence. Thus animals could not exist without plants to work up the crude inorganic materials into the complex and mobile molecules of protoplasm, which are alone suited for assimilation by the more delicate and complex organisation of animal life. Plants, on the other hand, could not exist without a supply of the carbonic dioxide, which is their principal food, and which animals are continually pouring into the air from the combustion of their carbonised food in oxygen, which supplies them with heat and energy. Thus nature is one huge aquarium, in which animal and vegetable life balance each other by their contrasted and supplemental action, and, as in the inorganic world, harmonious existence becomes possible by this due balance of opposing factors.