

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 like manner, our know- 

 ledge of the distribution of land and water, of the climatal conditions, oceanic 

 currents, etc., 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. 



