The Past in the Present 89 



grown into large eels which begin to become lusty. A 

 restlessness seizes them ; they descend the river, some- 

 times after squirming through a meadow to find it; 

 they reach the sea and swim out to a great distance 

 and a considerable depth in the Atlantic. There they 

 spawn and die; for the adults never return to fresh 

 water, and they never spawn except in the sea. 



This is but a glimpse of a long story, but it may 

 serve as a picturesque illustration of the outlook of 

 the new biology. We might refer, for instance, to the 

 interesting "tropism" of the elvers as they push 

 their way up the river, often against considerable 

 obstacles, adjusting their body automatically so that 

 the pressure of the current on the two sides is always 

 the same. But for our present purpose it is more use- 

 ful to refer to the restlessness of the full-grown eels 

 which sets them a-roving. It has been pointed out that 

 when the reproductive organs of the fish become ripe, 

 they exert an influence on the everyday routine of 

 chemical processes that go on in the laboratory of 

 the eel's body. There are perhaps internal secretions 

 to which we shall afterwards refer; in any case the 

 composition of the blood changes ; the proportion of 

 carbonic acid increases, and this, acting on the 

 nervous system, provokes restlessness. From the 

 vague words we are forced to use it is plain that the 

 study of the internal economy of the adolescent eel 

 has only begun; but the general idea has come to 

 stay, that periodic changes, such as the ripening of 

 the gonads in the body, have by-effects which lead 

 the creature to give new answers-back to external 



