Iviii THE ANIMAL KINGDOM. 



accounting for the epochs of the earth's history, and he discusses the results of such 

 catastrophic views on evokition. 



Returning to the consideration of the three factors or fundamental causes bringing 

 about a tendency to vai'iation, we may first consider the clianges in the relative distri- 

 bution of land and sea. Geological history is an epitome of the wide-spread and long- 

 continued changes in the shape of the continents, from the time when, as Laurentian 

 land-masses, there appeared but isolated nuclei of what are now the continents. Geol- 

 ogy shows that these primeval incipient continents were original centres of creation, 

 and that however contiguous continents may have borrowed one another's features, 

 whether to a limited or wide extent, yet, notwithstanding a nearly uniform tempera- 

 ture and climate, the evolution and specialization of life-forms went on throughout 

 the different growing continents, resulting in the zoo-geographical realms of the 

 present day. Here also should be taken into account the elevation of the Hima- 

 layas, the Cordilleras of America, and the Alps and other high mountain chains, 

 producing circumscribed areas, with different climates and other geographical fea- 

 tures. 



Finally came the Ice period, with the division of the eartli's temperature into tor- 

 rid, temperate, and frigid zones. The changes in the animals and plants resulting from 

 these events must evidently have brought about (1) the extinction of many older types, 

 tliose unfitted for the new conditions of life ; (2) the modification of others more 

 plastic and endowed with greater vitality, while (3) a few forms, such as Lingula, 

 Ceratodus, etc., endowed with still greater vitality, persisted from early times till now. 

 They were the sole survivors of changes in physical conditions and of a wreckage of 

 life-forms, whose remains fill the cemeteries of paleozoic, mesozoic, and tertiary times. 



Geological history also sliows that there have been jjeriods of long preparation, 

 marked by oscillations of continents, finally terminating in crises. Examples are the 

 accumulations of sediments, their upheaval, metamorphism, and conversion into the 

 Alleghanies, which marked the end of tlie paleozoic era in eastern North America. 

 The processes of continent-making went on in the eastern hemisphere, beginning at 

 the time when Eurojie was an archipelago and ending with the period when these 

 islands became united, and Europe and Asia were consolidated into a single continent. 



The crises in organic life, the origin, rise, culmination, and final extinction of types 

 of organic life, went hand in hand with these great changes in the physical geography 

 of our earth, as seen in the history of the trilobites, of the brachiopods, of the Neba- 

 lids, the Enryj)terids, the Dipnoans, .and the Labyrinthodonts, etc.; and among plants, 

 the Lepidodendrons, Calamites, Sigillarias, and other extinct forms. 



Finally, the embryological development and metamor])hosis of animals often have 

 a most significant me.aning, being condensed histories of changes which must have 

 occurred in the history of tlieir type in past ages. The generalized appearance of the 

 embryo is paralleled by the generalized condition of paleozoic types and their present 

 survivors ; the sudden assum])tion of special characters at or just before the time of 

 birth, is jiaralleled by the great specialization in form and structure which went on 

 throughout tlie world in the mesozoic and tertiary times, when forests of club mosses, 

 giant Equiseta, and synthetic, broad-leaved conifers, gave way to growths of modern 

 pines and oaks, beeches, willows, po])lars, mai)les, and other hard-wood trees ; while 

 among animals thousands of species of bony fishes, and tlie whole class of mammals, re- 

 placed the generalized quadru]5edal back-boned creatures which haunted the carbon- 

 iferous forests — growths of old-fashioned tree-ferns and club-mosses, with not a flower- 



