682 THE POPULAR SCIENCE MONTHLY. 



a different kind of irritability, or at least a conspicuous lack 

 of tlie 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 physi- 

 ological trait ; developed through ages of natural selection, in ac- 

 cordance with the laws of use and disuse, and here and there modi- 

 fied 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. 



^«» 



A FOSSIL CONTINENT. 



IF an intelligent Australian colonist were suddenly to be trans- 

 lated backward from Collins Street, Melbourne, into the flour- 

 ishing 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 sum- 

 mers or so, indefinitely (in geology we refuse to be bound by 

 dates), and would have landed him at last, to his immense aston- 

 ishment, 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, 



