68 THE ANATOMY OF INVERTEBRATED ANIMALS. 



sideration, however, will show that this classification of the 

 facts of distribution is essentially faulty, inasmuch as many 

 of the phenomena included under the second head are of the 

 same order as those comprehended under the first. Zoological 

 Distribution comprehends all the facts w T hich relate to the 

 occurrence of animals upon the earth's surface throughout 

 the time during which animal life has existed on the globe. 

 Therefore it embraces : 



First, Zoological Chronology, or the duration and order of 

 succession of living forms in time ; and — 



Secondly, Zoological Geography, or the distribution of life 

 on the earth's surface at any given epoch. 



What is commonly termed Geographical Distribution is 

 simply that distribution which obtains at the present epoch ; 

 but it is obvious that, at any given moment in their past his- 

 tory, animals must have had some sort of geographical distri- 

 bution ; and considerable acquaintance with the nature of that 

 distribution has now been obtained for all the epochs, the 

 nature of the living population of which has been revealed by 

 fossil remains. I do not propose to deal at length with either 

 branch of distribution in this place, but a few broad truths 

 which have been established may be mentioned. 



Geographical Distribution at the Present Epoch. — The 

 fauna of the deep sea (below five hundred fathoms) has been 

 shown, by the investigations of Wyville Thomson and his 

 associates of the Challenger, to present a striking general uni- 

 formity (in all parts of the world hitherto explored, in corre- 

 spondence with the general uniformity) of conditions at such 

 depths. 



With respect to the surface of the sea, the observations of 

 the same naturalists tend to establish a like uniformity of the 

 great types of foraminiferal life throughout the tropical and 

 temperate zones — with a diminution in the abundance of that 

 life toward the arctic and antarctic regions, where it appears 

 to be replaced by Eadiolaria and Diatomaceous plants. 



W r ith regard to higher organisms, the oceanic Hydrozoa 

 and the Ctenophora are undoubtedly very widely spread. It 

 is probable that they attain their maximum development in 

 warm seas, though the known facts are insufficient for the 

 definite conclusion. Sagitta and Appendicularia, with many 

 genera of Copepoda, Crustacea, and Pteropoda, are of world- 

 wide distribution ; and it is at present doubtful whether any 

 well-marked provinces of the ocean can be defined by the oc- 



