300 Geograpliical Distribution of Plants 



figured" I can only summarise the conclusions of a technical but 

 masterly discussion. " The broad general features of the distribution 

 of continent and ocean can be regarded as the consequences of simple 

 causes of a dynamical character," and finally, "As regards the contour 

 of the gi-eat ocean basins, we seem to be justified in saying that the 

 earth is approximately an oblate spheroid, but more nearly an 

 ellipsoid Avith three unequal axes, having its surface furrowed 

 according to the formula for a certain spherical harmonic of the 

 third degi'eeV' and he shows that this furrowed surface must be 

 produced " if the density is greater in one hemispheroid than in the 

 other, so that the position of the centre of gravity is eccentric^." 

 Such a modelling of the earth's surface can only be referred to a 

 primitive period of plasticity. If the furrows account for the great 

 ocean basins, the disposition of the continents seems equally to 

 follow. Sir George Darwin has pointed out that they necessarily 

 " arise fi-om a supposed primitive viscosity or plasticity of the earth's 

 mass. For during this course of evolution the earth's mass must 

 have suffered a screwing motion, so that the polar regions have 

 travelled a little from west to east relatively to the equator. This 

 affords a possible explanation of the north and south trend of our 

 great continents*." 



It would be trespassing on the province of the geologist to pursue 

 the subject at any length. But as Wallace^, who has admirably 

 vindicated Darwin's position, points out, the "question of the per- 

 manence of our continents... lies at the root of all our inquiries into 

 the gi'eat changes of the earth and its inhabitants." But he proceeds: 

 "The very same evidence which has been adduced to prove the 

 general stability and permanence of our continental areas also goes 

 to prove that they have been subjected to wonderful and repeated 

 changes in detail^." Darwin of course would have admitted this, for 

 with a happy expression he insisted to Lyell (1856) that "the 

 skeletons, at least, of our continents are ancient I" It is impossible 

 not to admire the courage and tenacity with which he carried on the 

 conflict single-handed. But he failed to convince Lyell. For we 

 still find liim maintaining in the last edition of the Princijjles: 

 "Continents therefore, although permanent for whole geological 

 epochs, shift their positions entirely in the course of ages*^." 



Evidence, however, steadily accumulates in Darwin's support. 



' Report of the 11th Meeting of the British Association (Leicester, 1907), London, 1908, 

 p. 431. 



"- Ibid. p. 436. ' Ibid. p. 431. 



•» Encycl. Brit. (9th edit.), Vol. xxiii. "Tides," p. 379. 



» Inland Life (-and edit.), 1895, p. 103. " Loc. cit. p. 101. 



' More Letters, ii. p. 135. 



8 Lyell'fi Principles of Geology (11th edit.), London, 1872, i. p. 258, 



