INTRODUCTION 7 
seas and land areas. Deep-sea, shallow water, and littoral deposits are 
readily distinguishable by means of their fossil organisms. By fossils, also, 
even the climatal conditions of former periods are recorded with great 
fidelity. The luxurious and uniform development of cryptogams over the 
face of the globe during Carboniferous time presupposes a warm, moist 
climate, little varying with latitude; tropical dicotyledons occurring in the 
Cretaceous and Tertiary deposits of Greenland, or coral-reefs extending into 
high latitudes during the Palaeozoic era, prove with equal certainty the pre- 
valence of a milder climate and higher oceanic temperature in earlier times ; 
while again, the remains of reindeer, the lemming, musk-ox, polar fox, and 
other arctic animals in the diluvium of Central Europe testify to a period of 
glaciation with reduced mean annual temperature. 
The geographical distribution of fossil organisms proves that the regions 
and provinces occupied by recent plants and animals are to a certain extent 
identical with those existing in the Tertiary, and that life has been subject to 
the same distributional laws in the past as in the present. Nearly all recent 
forms are the obvious descendants of extinct creatures which formerly 
occupied the same region. For example, the fossil mammals, birds, and 
reptiles of Diluvial time in Europe, Asia, Australia, North and South America, 
are scarcely distinguishable from forms now inhabiting the same continents. 
The old ancestral homes of marsupials and edentates were perpetuated in 
Australia and South America until as recently as the Diluvial epoch, and 
during the Tertiary, Europe, Asia, and America formed but a single zoological 
province, inhabited by the ancestors of forms now living in the northern 
hemisphere. An understanding of the physical conditions which have 
governed the perpetuation of recent plants and animals in their respective 
provinces (succession of similar types) would be utterly impossible without a 
knowledge of their distribution in former times. In lke manner, our know- 
ledge of the distribution of land and water, of the climatal conditions, oceanic 
currents, ete., of earlier periods depends chiefly upon evidence derived from 
fossils. 
Palaeontology and Embryology (Ontogeny).—To trace living plants 
and animals through all stages of growth from the egg upward to maturity, 
and thence on to final dissolution, is the task of Embryology or Ontogeny. 
At the present moment, botanists and zoologists are devoting their most 
scrutinising attention to embryological investigations, which latter accordingly 
exert a powerful influence on the progress of biology, and particularly on the 
classification. The fact that every individual, species, and genus of a whole 
group of plants and animals passes through nearly the same course of 
development, at least in the primary stages, and that all embryoes belonging 
to a given order or class resemble one another so closely, up to a certain 
stage, that they cannot be told apart, has revealed unexpected affinities among 
forms differing very considerably in the adult stage. Cirripedes, for example, 
which were formerly mistaken for shell-bearing mollusks, develop from the 
same Nauplius-larvae as the Copepoda, Phyllopoda, and Ostracoda, although 
the mature individuals belonging to these orders of crustaceans possess 
but little common resemblance. Likewise, the whole group of vertebrate 
embryoes can hardly be distinguished from one another in the earliest stages, 
and only very gradually assume the characteristic features pertaining to class 
and order. 
