INTRODUCTION 7 



their fossil organisms. By fossils, also, even the climatal conditions of 

 former periods are indicated 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 Paleozoic era, prove with 

 equal certainty the prevalence 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 ancestral homes of marsupials and edentates were perpetuated in 

 Australia and South America until as recently as the Diluvial epoch, and 

 during the later 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 prevailing climatal conditions, 

 oceanic currents, etc., of earlier periods depends chiefly upon evidence derived 

 from fossils. 



Paleontology and Embryology. — To trace the development of living 

 plants and animals through all stages from the one-celled egg onward to 

 final dissolution, is the task of Embryology and 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 embryos 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, Branchiopoda and Ostracoda, although the mature individuals 

 belonging to these Orders of crustaceans possess but little common resemblance. 

 Likewise, the whole group of vertebrate embryos can hardly be distinguished 

 from one another in the earliest stages, and only very gradually assume the 

 characteristic features pertaining to class and order. 



The results of embryological inquiry have a most important bearing on 

 paleontology. Numerous fossil forms are known, which, in comparison with 

 recent related organisms, exhibit embryonic, or at least larval or adolescent 



