170 FISH AS PA THERS. 



or caso in this instance is prolonffed at the edge into a 

 kind of broad wing on either side, so that it exactly 

 resembles one of the large flat leaves of the Antarctic 

 fucus in whos3 midst it lurks. It forms the high-water 

 mark, I fancy, of protective resemblance amongst eggs, 

 for not only is the margin leaf-like in shape, but it is 

 even gracefully waved and fringed with floating hairs, as 

 is the fashion with the expanded fronds of so many 

 among the gigantic far-southern sea-weeds. 



A most curious and interesting set of phenomena are 

 those wliich often occur when a group of fishes, once ma- 

 rine, take by practice to inhabiting freshwater rivers ; or, 

 vice- versa, when a freshwater kind, moved by an aspira- 

 tion for more expansive surroundings, takes up its 

 residence in the sea as a naturalised marine. Whenever 

 such a change of address happens, it usually follows that 

 the young fry cannot stand the conditions of the new 

 home to which their ancestors were unaccustomed — we 

 all know the ingrained conservatism of children — and so 

 the parents are obliged once a year to undertake a 

 pilgrimage to their original dwelling-place for the 

 breeding season. 



Extreme cases of terrestrial animals, once aquatic in 

 habits, throw a flood of lurid light (as the newspapers say) 

 upon the reason why this should be so. For example, 

 frogs and toads develop from tadpoles, which in all 

 essentials are true gill-breathing fish. It is, therefore, 

 obvious that they cannot lay their eggs on dry land, 

 where the tadpoles would bo utiable to find anything to 

 breathe ; so that even the driest and most tree-haunting 

 toads must needs repair to the water once a year to deposit 



