286 KANSAS CITY REVIEW OF SCIENCE. 
spontaneous tendency, except within the limits set by its growth and the law of 
its periodical changes. There may, however, be equilibrium more or less stable. 
I believe all attempts hitherto made have failed to account for the fixity of cer- 
tain, nay, of very many, types throughout geological time, but the mere consider- 
ation that one may be in a more stable state of equilibrium than another, so far 
explains it. A rocking stone has no more spontaneous tendency to. move than 
an ordinary boulder, but it may be made to move with a touch. So it probably 
is with organisms. But if so, then the causes of variation are external, as in many 
cases we actually know them to be, and they must depend on instability or 
change in surroundings, and this so arranged as not to be too extreme in amount 
and to operate in some determinate direction. Observe how remarkable the unity 
of the adjustments involved in such a supposition ; how superior they must be to 
our rude and always more or less unsuccessful attempts to produce and carry 
forward varieties and races in definite directions. This cannot be chance. If it 
exists it must depend on plans deeply laid in the nature of things, else it would 
be most monstrous magic and causeless miracle. 
Another caution which a palseontologist has occasion to give with regard to 
theories of life, has reference to the tendency of biologists to infer that animals 
and plants were introduced under embryonic forms, and at first in few and im- 
perfect species. Facts do not substantiate this. The first appearance of leading 
types of life is rarely embryonic. On the contrary, they often appear in highly 
perfect and specialized forms, often however of composite type and expressing 
characters afterwards so separated as to belong to higher groups. Again, we are 
now prepared to say that the struggle for existence, however plausible as a theory^ 
when put before us in connection with the productiveness of animals and the few 
survivors ot their multitudinous progeny, has not been the determining cause of 
the introduction of new species. 
No conclusions of geology seem more certain than that great changes of 
climate have occurred in the course of geological time, and the evidence of this 
in that comparatively modern period which immediately preceded the human age 
is so striking that it has come to be known as pre-eminently the ice-age; while in 
the preceding tertiary periods, temperate conditions seem to have prevailed even 
to the pole. Of the many theories as to these changes which have been pro- 
posed, two seem at present to divide the suffiages of geologists, either alone or 
combined with each other. These are (i) the theory of the precession of the 
equinoxes in connection with the varying eccentricity of the earth's orbit, advo- 
cated more especially by Croll; and (2) the different distribution of land and 
water as affecting the reception and radiation of heat and the ocean currents, a 
theory ably propounded by Lyell, and subsequently extensively adopted either 
alone or with the previous one. One of these views may be called the astronom- 
ical, the other the geographical. I confess that I am inclined to accept the 
second or Lyellian theory for such reasons as follows: (i) Great elevations and 
depressions of land have occurred in and since the Pleistocene, while the alleged 
astronomical changes are not certain, more especially in regard to their probable 
