Vlll INTRODUCTION. 



sibility, of many other and more perfect structures 

 having been formed elsewhere at the same early period 

 of the world's history. The maxim " de non apparen- 

 tibus et non existentibus eadem est ratio " is scarcely 

 applicable to geological cases of this nature. 



The conditions which exist in one part of the sea-bed 

 are often quite different in another part. The late Pro- 

 fessor Forbes, in his valuable Report to the British 

 Association in 1843 on the Invertebrata of the ^Egean, 

 stated his belief that the zero of animal life was pro- 

 bably about 300 fathoms, because his dredgings in that 

 sea at a depth of 230 fathoms yielded but very few 

 species. But in other tracts of the ocean living animals 

 of various kinds have been repeatedly obtained from far 

 greater depths. Our knowledge of abyssal life is only 

 checked by the difficulty of such explorations and by 

 the imperfect nature of our means of discovery. It is 

 a high and worthy object of the naturalist's ambition, 

 and by no means devoid of general interest. 



" There is a magnet-like attraction in 

 These waters to the imaginative power 

 That links the viewless with the visible, 

 And pictures things unseen." 



Speculations of this kind were not unknown to the 

 ancients. In the ' Halieutica ' of Oppian, written nearly 

 seventeen centuries ago, it is stated that no one had 

 found the bottom of the sea ; and that the greatest depth 

 ascertained by man was 300 fathoms, where Amphi- 

 trite had been seen. But this grand discovery does 

 not seem to have satisfied the poetical philosopher, and 

 he enters into a long disquisition as to the many other 

 wonderful things that may be concealed in the recesses 

 of the boundless ocean adding, however, 



" oXi'yos 5e voos pepoTreffffi rat 



