Vol. XXIV. No. 6.] 
POPULAR SCIENCE NEWS. 
83 
and tlie brilliant colors and patterns of male birds 
and buttertlies, to a process that he called "sexual 
selection." Sexual selection, applied to the devel- 
opment of the exceptional strength, size, and 
activity of the male, together with the possession of 
special offensive and defensive weapons, is admitted 
by Mr. Wallace to be a real power in Nature. But 
he is unable to follow Darwin into the extension 
of it to include the direct action of female choice or 
preference. The display of plumes and crests and 
gorgeous color, in so far as it is the outward and 
visible sign of the maturity and vigor of the male, 
is undoubtedly attractive to the female. It is going 
too far, Mr. Wallace believes, to credit her with 
icsthetic emotions and artistic tastes strong enough 
to cause her to choose her mate on account of 
minute differences in the forms, colors, or patterns 
of their ornamentation. Moreover, it has been 
proved that among butt(?rflies and moths, where the 
display of color and ornamentation in the male is 
very great, the female exerci.ses no choice at all. 
As a substitute for this theory of female choice, 
Mr. Wallace elaborates one for which he acknow- 
ledges his indebtedness to a posthumous work by 
Mr. Alfred Tylor. Of this, the underlying principle 
is, the general dependence of diversified coloration 
on structure ; the tracts in which distinct develop- 
ments of color appear being marked out by the 
chief divisions of the skeleton in vertebrates and 
by the segments in the annulosa. Further than 
this, colors are shown by Mr. Wallace to vary in 
brilliancy, according to the degree of muscular and 
nervous development of the part on which they 
appear, reaching among birds a marvellous degree 
of perfection on the frills and crests and jewelled 
shields of the tropical humming-birds; in the 
resplendent eyespots on the elongated tail-coverts 
of the peacock ; on the enormously expanded wing- 
feathers of the argns-pheasant, and the magnificent 
shoulder plumes of the birds of paradise. Mr. 
Wallace would derive not only the color but the 
ornamental appendages themselves from an excess 
of strength, vitality, and growth-power, and to the 
same cause attribute their display at all periods of 
nervous or sexual excitement. It is in the tropics 
that this surplus of vital force is best able to expend 
itself without injury. In the tropics are concen- 
trated forms of life driven from temperate regions 
by glacial periods of extreme severity. Here the 
luxuriant vegetation affords abundant food and 
perennial shelter, and the course of development 
has been almost unbroken and unchecked from 
remote geological times. The tropics, therefore, 
are the paradise of the animal world, and, entering 
it with Mr. Wallace, we see in its gorgeous occu- 
pants a culmination of the marvel and mystery of 
animal color — for a marvel and mystery it must 
remain, even though each separate hue has been 
produced through the agency of natural selection. 
[Original in PopttUtr Science yews J 
THEORIES ON THE FORMATION OF THE 
EARTH. 
BY JOSEI'II W.\LL.'^CE. 
The searcher after truth has many sources to 
draw irom while making up evidence on the great 
antiquity of the world. True, he will meet some 
sciolists with a smattering of science, who make 
extravagant demands for the formations and depos- 
its, which can be accounted in more ways than 
one. Theories keep pace with the advancement of 
science; some, in their widest application, demand 
such long periods of time that natural science 
cannot grant them: Geology is also in an unsettled 
state on the time question, owing to the theory now 
rapidly gaining ground that the rocks were formed, 
not by mechanical force, but by chemical agency 
superadded. How does geology or natural science 
in its present state explain the origin of heaven, or 
even that of our globe.' 
Science is not positive on this question. All 
modern geologists of note .icknowledge that all the 
hypothesis about the former history of the earth 
must be based upon its present condition — on the 
forces now at work, on the laws which now exist; 
that all hypothesis must be rejected which begins by 
assuming that formerly difVerent laws of Nature 
were in force. Still, it cannot be denied the 
scientist the question: Have the causes which are 
now at work always existed in a like manner, with 
equal force and to the same extent.' This suppo- 
sition is held by Sir Charles Lyell ; others, however, 
say : May we assume that such causes have worked 
ditferently at different times, — at primitive times 
more powerfully than they do at present? Accord- 
ing to the first theory, the course of the earth's 
history would have been comparatively quiet; and 
according to the second, its development in ancient 
times would have been interrupted by great catas- 
trophies, revolutions, and convulsions. 
It is extremely arbitrary to assume that all geo- 
logical phenomena have been brought about by 
causes similar to those which are at work in our 
days, and that those causes never possessed greater 
force than they have had since the present order of 
things. Nature does not now work as she did for- 
merly, for the circumstances are no longer the same. 
We see the great series of Neptunian deposits 
divided off into a certain number of groups. This 
leads us to the thought of a series of sudden and 
violent catastrophies, each one of which was able to 
change the form of the seas and the course of the 
rivers over vast tracts, which were separated from 
one another by periods of comparative quiet. (For 
full text see Leonhardt, Geology, II., page 70.) 
Sir R. Murchison, speaking in 1865, (see Athen- 
ceum, September 16, 1S65, page 376), says; "I 
adhere (in opposition to Ramsay, Jukes, and 
Geikie) to my long-cherished opinion as to the 
great intensity of power employed in the production 
of dislocations of the earth. • • • Admiring 
the Huttonian theory, I maintain that such reason- 
ing is quite inadequate to explain the manifest 
proofs of convulsive agency, which abound all over 
the crust of the earth. * ♦ * I reject as an 
assumption which is at variance with the number- 
less proofs of intense disturbance, that the mechan- 
ical disruptions of former periods and the overthrow 
of entire formations, as seen in the Alps and many 
mountain chains, can be accounted for by any 
length of existing causes." 
These conflicting theories have established Con- 
vulsionists, or Catastrophists ; Quietists, or Uni- 
formitarians, as they are called ; also Neptiinists, 
and Plutonists, or Vulcanists ; the latter denote 
another deeply rooted opposition of parties. The 
theory formulated after this depends on the relative 
influence which is accorded to water and to fire, 
respectively, in the formation of the earth. Accord- 
ing to the Neptunian theory, the universe went 
forth from the water. All elements and constituent 
parts of the earth were originally dissolved in 
water. Through pressure, chemical precipitate, 
and crystallization, the primitive mountains se- 
creted themselves in the course of time from the 
pulp-like mass ; then, through further alluvials and 
deposits, the transition rocks and alluvial earth 
arose, until, finally, through the destruction in the 
mountain layers, as also through inflammation of 
combustible matter in the interior of the earth, and 
the thousands of other changes, the present surface 
of the earth formed itself. 
Such was the theory in vogue for quite a long 
time, but in the middle of the present century it 
was supplanted by an opposite view. Experience 
teaches us, however, that the heat of the earth 
increases the deeper we penetrate, and both the 
ignivomous mountains and hot fountains which 
break forth from the interior of the earth, led to the 
conclusion that we have to ascribe to fire a greater 
influence than to water towards the formation of 
the earth. Those who adhere to the I tter opinion 
do not altogether deny the effects of water, but they, 
give preference to fire; and we can say that the 
largest number of geologists in our days accept the 
Vulcanian theory. 
The Vulcanian theory was ably defended by Seoth 
Hutton (1726-179:;) and by a great number of Ger- 
man geologists. However, we have to remark here 
that very recently this entire hypothesis was greatly 
shaken by Ludwig, Lyell, Wagner, and Winkler. 
The adherents of the Vulcanian theory explain 
the formation of the earth in this way : They 
accept that the elementary parts of creation were 
diffused in the universe in the form of vapor or gas ; 
the siinple parts attracted each other, and began to 
turn themselves around a coinmon central point; 
hereby the globe began to form itself, which, at the 
beginning, was a very extensive gas globe, some- 
thing like a glowing ball ; through the emission of 
heat, this igneous mass became gradually harder or 
more condensed, until, finally, a firm crust, like a 
stiff bark, arose on the surface. These were the 
primitive rocks which appeared on the surface of the 
earth, and, as the hot mass in the interior moved 
and surged to and fro, they broke through the 
exterior crust from time to time. Thus empty 
spaces formed themselves on the surface of the 
earth, while its exterior crust grew gradually cooler, 
and deep chasms and depressions took place. These 
mountains which formed the primitive rocks are 
marked down as the first stage in the earth's surface, 
and the origin of metals is laid at this period. 
A second stage in the development of the earth 
into its present state is the period during which the • 
water exercised its influence on the formation of the 
earth's surface. The gases which moved in the air 
condensed themselves more and more on the cooled 
earth, and formed a great ocean, which covered the 
entire surface. These waters, which were boiling 
hot, contained various elements*that affected a part 
o( the formation of the surface. Consequently, 
various deposits, elevations and depressions of the 
ground took place, and, through the activity of the 
water, the so-called mountain chains formed them- 
selves. 
The cooling of the earth and deposits of vapors 
on the earth's surface continued in later periods, 
until, finally, the terhperature sank so low that veg- 
etation could form itself upon the earth- The 
climate, at first, was tropical, which spread itself 
equally over the entire surface. Plants and animals 
came forth luxuriantly and in fulness of life; the 
great ^evolutions which changed the shape of the 
earth's surface and again destroyed whole species of 
fauna and flora, gradually ceased ; the temperature 
finally sank so low that ice formed itself in different 
localities, and now the earth does not emit more 
heat than what it receives from the sun. Thus, in 
millions of years, the surface of the earth arrived at 
its present condition, and the interior is yet in an 
igneous fusion. 
As to the length of time for the formation of the 
earth, G. Bichoft" says 353,000,000 years must have 
elapsed, and Pfaff thinks it likely that the solidify- 
ing of the earth's crust took no less than 20,000,000 
years, and the formation of the earth to its present 
state not more than 400,000,000 years. The thought 
alone of this immense number of years is enough 
to stagger and bewilder the most profound thinker. 
