eo . NATURE 
perceptible but unceasing change of the level of the seas, in con- 
sequence of the phenomena of subsidence and elevation of the 
land, lands at one time united have been divided, watercourses 
which communicated have been separated, thus accounting for 
the fact that fishes of the same species are found in different 
rivers, that islands are tenanted by the same mammals as 
the continents. England has been united to Europe at two 
different times ; at a certain epoch our continent must have been 
united by land to N. America. The Sourh-sea Islands are the 
remains of what was at one time a single land; so in the Indian 
Ocean land has at one time stretched along the South of Asia 
from Sunda to Africa; this great continent which Sclater has 
called Lemuria, on account of the apes which were peculiar to 
it, is probably the cradle where the human race was developed 
from the anthropoid apes. Mr. Wallace has proved that the 
Malay Archipelago consisted of two entirely different parts: one, 
comprehending Borneo, Java, and Sumatra, was united to Asia 
by the peninsula of Malacca, while the other, comprehending 
the Celebes, the Moluccas, New Guinea, the Salomon Isles, &c., 
was immediately attached to Australia, 
Another cause which has favoured the dispersion of species 
all over the globe, was the uniformity of temperature which pre- 
vailed up to the tertiary geological period. Previous to the 
freezing of the polar regions, species found everywhere a climate 
equally warm and agreeable, favourable to migrations in all 
directions ; since that period, on the contrary, a new difficulty of 
existence has arisen, —organisms have to undergo acclimatisation ; 
those which have the power of adapting themselves to the lower 
temperature of regions at a distance from the equator, have been 
transformed by selection into new species ; while those which 
have found such adaptation impossible, have been compelled, 
under pain of extinction, to remove to more favourable climates. 
When, at a later period, occurred that strange phenomenon—of 
which, as yet, no satisfactory explanation has been given— 
known as the Glacial Period, animals and plants were compelled 
to migrate anew ; the living popu'ation of the earth, condensing 
itself between the tropics, a terrible struggle for existence took 
place between the old inhabitants of these regions and those that 
fled thither for refuge ; many species were bound to disappear, 
while many new ones were originated. There is still another choro- 
logical phenomenon which is to be accounted tor by the glacial 
period, viz., the resemblance of many of the inhabitants of moun- 
tains to those of the Polar regions ; as those animals and those 
plants are not found in the intermediate countries, it is absolutely 
necessary to suppose a migration which, considering the habits 
of these creatures, could only have taken place at the glacial 
epoch, It is probable that at this period the gentians, the saxi- 
frages, the Polar hare and fox, inhabited the central part of 
Europe ; but as the temperature rose, some of these creatures 
retired towards the north, while the remainder found a refuge 
upon the summi's of the European mountains. 
When planis or animals migrate to new regions, they are sub- 
jected to new conditions of existence to which they must adapt 
themselves. The new climate, new food, relations with new 
organisms, all this oblizes the emigrants to submit to mo- 
difications under pain of annihilation, and, as a consequence, 
to form new varieties or new species; it is in these 
circumstances, ia fact, that matural selection acts with 
the greatest intensity. In ordinary circumstances, indi- 
viduals which have changed breed with indivduals who 
have not changed, and the products of such crossings have 
a tendency to revert to the prim tive type; but when a migra- 
tion has taken place, when modified individuals are separated 
from the others by mountains or by seas, they can no longer 
interbreed, and this isolation insures the preservation of the 
newly acquired forms. It is of course evident that these con- 
siderations apply only to species in which the sexes are separate. 
There still remain three other chorological phenomena which 
furnish an important proof of the truth of the evolution theory. 
There is first the likeness of form, the family resemblance which 
exists among the local species characteristic of each region, and 
the extinct and fossil spe-ies of the same region ; in the second 
the no less striking family resemblance which exists among the 
inhabitants of certain groups of those of the neighbouring con- 
tineats, whence the population of these islands must have come ; 
and lastly, the special character pres:nted by the cullective fauna 
and flora of the islands. All the facts adduced by Darwin, 
Wallace,* and Moritz Wagner,+ as weil as all those other facts 
* “ Malay Archipelago.” 
+ The “Darwinian ‘Theory and the Law of Migration of Organisms” 
Leipzig, 1868). 
which geographical and topographical dispersion of organisms 
present to us are simply and completely explained by the theory 
of selection and migration, while it would be impossible to 
explain them without it. 
Paleontology 
Thanks to the theory of evolution, the natural classification of 
animals and plants, which was previously Only a record of names 
for arranging the different forms in an artificial order, or a record 
of facts expressing summarily the degree of resemblance among 
them, tends to become the genealogical tree of organisms. 
In order to construct it the student has only to combine the 
data furnished by the three parallel developments referred to 
above—the palzeontological development, the embryological de- 
velopment, and the systematic development in the order of per- 
fection or of comparative anatomy. The writer jin the Reve. 
Scientifique here gives a table presenting a view of the geological 
and palzeontological doctrines of Haeckel. Between the stages 
generally admitted by geologists, Haeckel intercalates others 
which he calls inferior or intermediate stages in relation to the 
superior stages. Haeckel accepts completely the system of gra- 
dual and continuous evolution as propourided by Lyell, and re- 
jects the system of sudden catastrophes which has been advocated 
by Cuvier and his disciples. He places the probable appearance 
of man in the Miocene, and his certain existence in the Pliocene. 
Many attempts have been made to determine approximately how 
many thousand years each geological period has lasted ; these 
conjectures are principally framed on the relative thickness of 
the different beds, The total thickness of the Archzolithic or 
Primordial beds, in which Haeckel includes the Laurentian, Cam- 
brian, and Silurian, is 70,000 ft. ; that of the Primary, from the 
Devonian to the Permian, 42,000 ft. ; that of the Secondary, 
15,000 ft. ; that of the Tertiary, 3,000 ft. ; while the thickness 
of the beds of the ‘‘ Anthropolithic ” or Quaternary age is only 
from 500 to 7ooft. From these figures, the following relative 
duration of the successive ages may be deduced :— 
Primordial Age : e 5 53°6 
Peimary Fy 5 ‘ . 32°1 
Secondary ,, : ‘ : 11'S 
Tertiary i3 F , 2°3 
Quaternary ,, . . . o’5 
Thus the Primordial age has existed longer than the other four 
put together. As to the number of centuries or of millenniums 
necessary for the deposition of one bed only one foot thick, that 
depends on circumstances so yariable that it is impossible to give 
any measure: it is longer in the depths of mid-ocean, in the 
beds of very long rivers, in lakes which receive no affluents ; it is 
shorter on the sea-margins, at the mouths of great rivers whose 
course is long and straight, in iakes which receive many tributary 
streams. It results from such considerations that every estimate 
of the duration of a geological epoch must be relative. 
It will be necessary, moreover, to take into consideration, 
elevations and depressions of the ground, which, according to 
Haeckel, will be alternative, and will correspond to the minera- 
logical and paleontological differences which exist between 
two systems of beds and between two formations of these systems. 
When a certain region, after having remained for many thousand 
centuries beneath the water, emerges for a certain tim’, and is 
again submerged, it will be readily admitted that the bed which is 
deposited after such an interval ought to present characteristics 
different from those of the lower bed: for time is bound to 
accomplish change of all organic and organic conditions. This 
theory has been disputed by Huxley, who finds it inconsistent 
with the existence of a large number of beds, in which are found 
united organic forms, holiing a middl2 place between those of 
adjacent formations ; the English naturalist adduces, for example, 
the beds of Saint Cassian, in which are found mingled the forms 
of the primary and secondary formations. 
It is certain that even yet our knowledge of paleontology is 
very imperfect, and far from enabling us to write, with anything 
like exactness, the history of the production of organic species. 
We know with what difficulties this study is surrounded. The 
fossil remains of the most remote ages appear to have been 
destroyed by the great heat of the lower beds in which they were 
deposuied. £oz00n Canadense is the only fos,il which has hitherto 
been found in the formations of the Laurentian period ; while 
the beds of carbon and of crystallised lime(graphite and marble) 
give us the assurance that in them have existed animal and 
vegetable petrifactions. Another difficulty lies in the fact that 
hitherto the field of geological exploration has been very re- 
| May 8, 1873 
