136 THE INFLUENCE OF INANIMATE SUEEOUNDINGS. 



banks of streams. But what was far more striking in tlieso 

 islands was the total absence of all periodicity in the life of the 

 sea animals, particularly the Invertebrata ; among these I could 

 not detect a single species of which I could not at all seasons 

 find fully grown specimens, young ones, and freshly deposited 

 eggs. Even in cold seas periodicity is far more often eliminated 

 than is commonly supposed. Nordmann tells us that he has 

 found the eggs of a sea mollusc {Tergipea) at all seasons, even 

 in midwinter, when the temperature of the water was only at 

 a few degrees above freezing-point ; Mobius *' states that eggs 

 of MoUusca and Worms were found at all seasons of the year 

 in the Baltic, and I hope that Dohrn's zoological station at 

 Naples may ere long furnish us with a long list of species 

 which may be observed the whole year round in all the stages at 

 once of growth, larvae, and eggs. So far as I can judge from my 

 own observations, which of course are by no means conclusive, 

 the Mollusca manifest the least periodicity in their reproductive 

 powers. 



The influence of an equable temperature on the iniabitants 

 of the sea is displayed, too, in another manner. It is known 

 that in our northern seas the daily variations of temperature 

 on the shore, or even at some depth, are not inconsiderable, while 

 in tropical seas the variation at the surface even between winter 

 and summer heat is not neai-ly so great — at the Philippines, for 

 instance, not more than 2° centigrade ; so small a variation, as 

 has been shown by Meyer and Mobius in a remarkable work,'^ 

 occurs only at a great depth in northern seas. It is to this 

 apparently that we must refer the fact that many genera of sea- 

 creatures which are known as boreal forms live in the north at 

 great depths, while in tropical seas they live very near the 

 surface. ^^ In my monograph on the Holothuridte I have shown 

 that a great number of genera which we had been accustomed 

 to regard as typically boreal were found also in the Philippine 

 seas, and lived there at a moderate depth, while in the northern 

 seas they were found only at very considerable depths. The 

 same seems to be the case with legard to many animal forms 

 which are now found at the bottom of the Atlantic, and which 

 may be regarded as survivors from a long past geological period ; 



