the Distribution of Life and of Rocks. 411 
the equator, the least organized at the poles. And since species 
diffuse themselves in the direction of least resistance, it will be 
along lines where the heat is uniform; so that homozoic belts 
(but for disturbing causes) will correspond with parallels to the 
equator. 
The present distribution of land and water, and the geological 
evidences of its mutations, show that species are compelled to 
migrate north and south (as well as east and west), and so be- 
come subject permanently to different degrees of the sun’s 
- energy (and its product food), which, as was seen, cannot but 
produce permanent changes in their organization. Hence it fol- 
lows that the same set of causes which introduce new rocks is 
also an instrument in introducing new species and new types, 
by changing the area of life. 
21. When a portion of the sea-bottom is elevated so as to be- 
come land, the life which covered that area is displaced ; that 
is to say, the group of life, from being continuous over an area, 
comes to surround a space which, so far as marine life is con- 
cerned, is a desert, The method by which this is accomplished 
is, that mountain ridges make divisions in the life-province; and 
then, just as the waters drain down the valleys of the land con- 
verging to an estuary, so also do the organisms drain off and 
converge with the separation of the waters: hence, but for dis- 
turbing causes, life will always be most abundant in species 
around the seaward terminations of the great areas of drainage. 
But, by the division of a group of life in this way, it happens (if 
the elevation is carried to a great extent) that each part of the 
old life-group becomes mixed with the new group on which it 
is compelled to encroach. If land already existing is still fur- 
ther upheaved, it can only happen that the life “will migrate 
further away ; so that the fauna which in one age occupied a 
given sea-bottom comes in a succeeding age to occupy an adja- 
cent area. 
22. Ifa portion of the sea-bottom is daiedliad: the life that 
covered it migrates away, following the shore as it recedes. 
And also if a portion of land is depressed so as to become sea- 
bottom, the life that covered the adjacent area migrates over it, 
and life of the present age becomes diffused in the succeeding 
age over an adjacent area without admixture with any new 
forms, except such as may be produced by the changed condi- 
tions. 
23. If elevation occurs so that a land-surface is enlarged, 
then the species already upon it migrate over the newly added 
area and down the mountain-sides, always diffusing most Ls i 
in the direction of least resistance. 
