a different kind of irritability, or at least a conspicuous lack of the normal kind, is denoted by such, habits of growth; but gravity, heredity, and anatomical peculiarities may be entirely responsible. Nor need the meaning of heliotropism in a theory of descent be seriously affected by the observations of Sachs upon certain roots which, although never normally in the light, showed marked heliotropic irritability when grown in illuminated water. In such cases a change in protoplasmic structure might easily have ensued after the change in life-conditions and before the manifestation of unexpected irritability. It is this which renders conclusions drawn from such data as Sachs had doubtful and, perhaps, fallacious.
Heliotropism, then, must be considered as a well-marked physiological trait; developed through ages of natural selection, in accordance with the laws of use and disuse, and here and there modified or altogether absent, as the needs of the organism chanced to demand. It is a result of irritability, and is usually manifested in connection with growth. As acknowledged above, it is still rather poorly understood, in its more recondite expressions; but, in general, it may justly be held to be a very complicated reaction in the department of molecular physics, or chemistry.
IF an intelligent Australian colonist were suddenly to be translated backward from Collins Street, Melbourne, into the flourishing woods of the secondary geological period — say about the precise moment of time when the English chalk downs were slowly accumulating, speck by speck, on the silent floor of some long-forgotten Mediterranean — the intelligent colonist would look around him with a sweet smile of cheerful recognition, and say to himself in some surprise, "Why, this is just like Australia." The animals, the trees, the plants, the insects, would all more or less vividly remind him of those he had left behind him in his happy home of the southern seas and the nineteenth century. The sun would have moved back on the dial of ages for a few million summers or so, indefinitely (in geology we refuse to be bound by dates), and would have landed him at last, to his immense astonishment, pretty much at the exact point whence he first started.
In other words, with a few needful qualifications, to be made hereafter, Australia is, so to speak, a fossil continent, a country still in its secondary age, a surviving fragment of the primitive world of the chalk period or earlier ages. Isolated from all the remainder of the earth about the beginning of the Tertiary epoch,