to the newer ones, and all forms are now included as varieties of one species. Over 200 varieties have been discovered in enormous numbers; one characteristic form being found in each horizon. The simpler unornamented shells are from the lowest layers; the most recent forms being identical with a form now living only in North America and the fresh-water lakes of China, which was formerly described as a distinct genus. This evidence was found since the publication of the "Origin of Species" in 1859, and renders the record less incomplete.

Now that men realise the value of Paleontology, more attention has been directed to the subject; for in all cases positive palæontological evidence may be implicitly trusted, although negative evidence is worthless.

THE EXTINCTION OF SPECIES.

When a species or group has once disappeared there is no reason to suppose that the same identical form ever reappears-i.e., its existence, so long as it lasts, is continuous.

The influence of the size of animals, and its bearing on Extinction of Species, is of the greatest possible interest and importance. Many zoologists hold the view, in support of which evidence is steadily increasing, that the primitive or ancestral members of each group were of small size. Thus, in the case of birds, on the whole small birds show more primitive conditions of structure than the larger members of the same group, and the first birds were probably smaller than Archæopteryx. Reptiles and mammals also show in their earlier and smaller types more primitive features than their larger descendants.

Again, in the pedigree of the horse, one of the most striking points is the progressive reduction in size met with as we pass backwards in time. The Pliocene Hipparion was smaller than the existing horse; the Miocene Mesohippus was about the size of a sheep; while the Eocene Eohippus was no larger than a fox. Not only is there good reason for holding that, as a rule, larger animals are descended from ancestors of smaller size, but there is also much evidence to show that increase in size beyond certain limits is disadvantageous, and may lead to destruction rather than to survival. It has happened several times in the history of the world, and in more than one group of animals, that gigantic stature has been attained immediately before extinction of the group, a final and tremendous effort to secure survival, but a despairing and unsuccessful one. The Ichthyosauri, Plesiosauri, and other extinct reptilian groups, the Moas, and the huge extinct Edentates, are well-known examples; to which before long will be added the elephants and the whales.

The same classification applies to both recent and fossil animals; the large divisions are the same, but many minor groups have become extinct. All existing groups are not known to have existed for all time, and many have certainly not done so. Still

no one primary division of the animal kingdom is entirely extinct: it is merely the subdivisions that have died out. The earliest origin of all the great groups is driven back to extremely remote times, to the Palæozoic period, and palæontology tells us nothing about the mode of origin of the great divisions of animals. Darwin says: "I look at the geological record as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved ; and of each page, only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may represent the forms of life which are entombed in one consecutive formation, and which falsely appear to us to have been abruptly introduced. On this view the difficulties above discussed are greatly diminished, or even disappear."

GEOGRAPHICAL DISTRIBUTION.

The explanation of the distribution of animals on land and in the sea is a subject of great importance, which I propose here to touch upon only as it is affected by palæontological evidence.

A

Much information has been collected on this subject by exploration and by systematic observations obtained by dredging expeditions. gradually growing conviction has arisen that we must not be content with mere facts, but must demand an explanation of these facts, and that this explanation is in our power to find. It is to Wallace that we are especially indebted for our knowledge of the geographical distribution of animals.

The nature of the problems we have to consider is best shown by examples, of which the following will serve our purpose.

A. CAMELIDÆ, or Camels, are an exceedingly restricted group, the majority of species now living in domestication.

1. Camelus is highly characteristic of hot, parched deserts, and is found in Sahara, Arabia, Persia, Turkestan, and Mongolia, as far as Lake Baikal. There are none now living perfectly wild. Of Camelus there are two kinds: the dromedary, found in Asia Minor and Africa, has one hump; the Bactrian camel, possessing two humps, is confined to Asia, and especially Central Asia, north of the Himalayas.

2. Auchenia is of smaller size, with slender legs, and has no hump. It is confined to the mountainous and desert regions of the southern part of South America, and is often found on rugged snow-clad slopes at great elevations. Of this group, Llama and Alpaca are entirely domesticated-the former being used as a beast of burden in Peru and Bolivia ; the latter is cultivated both for its wool and for its flesh. Vicuna, the smallest member of the group, is found at elevations of 13,000 feet and upwards, in the Andes of Peru, Ecuador, and Bolivia. Guanaco, an animal the size of a fallow-deer, is

distributed in the plains of Patagonia and Tierra del Fuego.

Camels are thus distributed over two areas, comprising the mass of two continents, divided by a great ocean; one area being north of the equator, the other south of it, and separated by half the circumference of the globe. They are animals of large size, and it is hardly possible for their existence to have been overlooked. Hence we may assume that their geographical distribution is known correctly.

This is a good example of the difficulties in accounting for geographical distribution, and of the way in which they may be met. Evolution tells us that close anatomical resemblances mean near kinship, and forbids us to contemplate the possibility of animals, agreeing in a number of important points, having come into existence independently.

The anatomical characters of camels are well marked; they have two toes-viz., the third and fourth, and walk on the palmar surface of the middle phalanx, not on hoofs. The sole of the foot is formed by broad integumentary cushions, and the nails are small and flattened. The stomach consists of a paunch with smooth lining, provided with two groups of water-cells with narrow mouths. The cervical vertebræ are peculiar, in that the canal for the vertebral artery pierces the arch of the vertebra, instead of the transverse process.

The theory of gradual modification of animals