YOUTH IN BIRDS AND LOWER ANIMALS 53 



bars and acquiring a silvery colour. In the spring of the third 

 year they go down to the sea as smolts, which display a much darker 

 and more mottled coloration than salmon. In the sea they 

 rapidly mature, becoming silvery all over and developing their 

 sexual organs. They then ascend the rivers to breed, and their 

 duration of youth is thus at least three years, although from the 

 great change of size, a smolt weighing only a few ounces and a 

 grilse four or five pounds, it has been supposed that the young 

 fish may remain more than a single year in the sea. The fresh- 

 water eels migrate to the sea to spawn and lay their eggs at great 

 depths. These hatch out into ribbon-shaped larvae with very 

 small heads. These little fish have been known as Leptocephali 

 for many years, the different kinds of them receiving different 

 specific names before it was discovered that they were the larvae 

 of different kinds of eels. The larva of the common eel, formerly 

 known as Leptocephalus brevirostris, grows rapidly until it becomes 

 about two and a half inches long, when it passes through meta- 

 morphosis and becomes transformed to a small eel, which, curiously, 

 is only about two inches long. These small eels leave the bottom 

 of the sea and come up towards the coast when they are about a 

 year old. They then enter fresh water, ascending the rivers in 

 great numbers, and at night migrating from stream to stream across 

 wet grass. They live for a number of years before they become 

 adult, the largest size to which the females attain being a little 

 over a yard, that of males being much less. Then the sexual organs 

 begin to develop, the process taking several months, duiing which 

 the eels cease to feed. They then migrate down to the sea, and 

 when they have reached deep water, probably more than a hundred 

 fathoms, spawning takes place and the eels die. This is a curious 

 instance, very unusual amongst vertebrate animals, but common 

 in insects where nearly the whole life of the animal may be occupied 

 by the period of youth. It seems to be the case that eels spawn 

 only once, and that however long they live, or whatever size 

 they attain, they must be regarded as still in the youthful period 

 until they have ceased to feed and have begun to spawn. 



All the vertebrate animals have a structure not remote from our 

 own, a nervous system consisting of a brain and spinal cord, and 

 organs of smell, sight and hearing essentially similar to our nose, 

 eyes and ears. Amongst them we are on familiar ground, and have 

 some reason to suppose that we can interpret their mental operations 

 and emotions with a sympathetic intelligence. The bond is most 



