404 



NA TURE 



[August 26, 1897 



dispersal are sometimes employed, not regularly in each genera- 

 tion, but at long intervals, as opportunity offers. 



Early migration by land is nearly always out of the question. 

 Walking, and still more flying, are difficult exercises, which call 

 for muscles of complex arrangement and a hard skeleton. A 

 very small animal, turned out to shift for itself on land, would 

 in most cases perish without a struggle. There might be just a 

 chance for it, if it could resist superficial drying, and were small 

 enough to be blown about by the wind (Infusoria, Rotifera, and 

 certain minute Crustacea), or if it were born in a wet pasture, 

 like some parasitic worms. 



We can define two policies between which a species can make 

 its choice. It may produce a vast number of eggs, which will 

 then be pretty sure to be small and ill-furnished with yolk. The 

 young will hatch out eaily, long before their development is 

 complete, and must migrate at once in search of food. They 

 will, especially if the adult is slow-moving or sedentary, be fur- 

 nished with simple and temporary organs of locomotion, and 

 will generally be utterly unlike the parent. The majority will 

 perish early, but one here and there will survive to carry on the 

 race. 



Or the parent may produce a few eggs at a time, stock them 

 well with yolk, and perhaps watch over them, or even hatch 

 them within her own body. The young will in such cases com- 

 plete their development as embryos, and when hatched, will 

 resemble the parent in everything but size. 



Which policy is adopted will largely depend upon the number 

 of the family and the capital at command. There are animals 

 which are like well-to-do people, who provide their children 

 with food, clothes, schooling, and pocket-money. Their for- 

 tunate offspring grow at ease, and are not driven to premature 

 exercise of their limbs or wits. Others are like starving families, 

 which send the children, long before their growth is completed, 

 to hawk matches or newspapers in the streets. 



In Biology we have no sooner laid down a principle than we 

 begin to think of exceptions. The exceptions may be apparent 

 only ; they may, when fully understood, confirm instead of dis- 

 turbing the general principle. But this rarely happens unless 

 the principle is a sound one Exceptio probat regulam ; it is 

 the exception which tests the rule, to give a new application to 

 an old maxim. 



Parasites form one group of exceptions to our rule. Whether 

 they pass their free stages in air, water or earth, whether their 

 hosts are marine, fluviatile or terrestrial, they are subject to 

 strange transformations, which may be repeated several times in 

 the sam^ life-history. The change from one host to another is 

 often a crisis of difficulty ; many fail to accomplish it ; those 

 which succeed do so by means of some highly peculiar organ 

 or instinct, which may be dropped as quickly as it is assumed. 

 The chances of failure often preponderate to such an extent that 

 an enormous number of eggs must be liberated. Even a brief 

 parasitism may produce a visible effect upon the life-history. 

 The young Unio or Anodon attaches itself for a short time to 

 some fish or tadpole. To this temporary parasitism is due, as I 

 suppose, the great number of eggs produced, and a degree of 

 metamorphosis, unusual in a fresh- water mollusk. 



The Cephalopoda, which are wholly marine, and the Verte- 

 brates, whatever their habitat, very rarely exhibit anything which 

 can be called transformation. Some few cases of Vertebrate 

 transformation will be discussed later. Cephalopods and 

 Vertebrates are large, strong, quick-witted animals, able to move 

 fast, and quite equal in many cases to the defence of themselves 

 and their families. They often produce few young at a time, 

 and take care of them (there are many examples to the contrary 

 among Cephalopods and Fishes). They are generally able to 

 dispense with armour, which would have indirectly favoured 

 transformation. 



Echinoderms, which are all marine, develop with metamor- 

 phosis. There is an interesting exception in the Echinoderms 

 with marsupial development, which develop directly, and give 

 an excellent illustration of the effect of parental care. 



Insects, which as terrestrial animals should lay a few large 

 eggs, and develop directly, furnish the most familiar and striking 

 of all transformations. I have already discussed this case at 

 greater length than is possible just now (Nature, December 

 19, 1895). I have pointed out that the less specialised insect- 

 larvre, e.g. those of Orthoptera, make a close approach to some 

 wingless adult insects, such as the Thysanura, as well as to 

 certain Myriopods. Fritz Mliller seems to me to be right in 

 saying that the larva; of non-metamorphic insects come nearer 



NO. 1452, VOL. 56] 



than any winged insect to primitive Tracheates. The trans- 

 formation of the Bee, Moth, or Blow-fly is transacted after the 

 stage in which the normal Tracheate structure is attained, and I 

 look upon it as a peculiar adult transformation, having little in> 

 common with the transformations of Echinoderms, Mollusks, or 

 Crustaceans. 



In the same way I believe that some Amphibia have acquired 

 an adult transformation. Frogs and toads, having already as- 

 tadpoles attained the full development of the more primitive 

 Amphibia, change to lung-breathing, tailless, land-traversing, 

 animals, able to wander from the place of their birth, to seek out 

 mates from other families, and to lay eggs in new sites. 



Medus£E furnish a third example of adult transformation, which 

 seems to find its explanation in the sedentary habit of the polyp, 

 which probably nearly approaches the primitive adult stage. 

 But here the case is further complicated, for the polyp still 

 proceeds from a planula, which is eminently adapted for locomo- 

 tion, though perhaps within a narrower range. We have two 

 migratory stages in the life-history. Each has its own ad- 

 vantages and disadvantages. The planula, from its small size, 

 is less liable to be devoured, or stranded, or dashed to pieces, 

 but it cannot travel far ; the medusa may cross wide seas, but it 

 is easily captured and is often cast up upon a beach in countless- 

 multitudes. 



Adult transformation may be recognised by its occurrence 

 after the normal structure of the' group has been acquired, and 

 also by its special motive, which is egg-laying and all that per- 

 tains to it ; the special motive of larval transformation is dis- 

 persal for food. 



The reproduction of the common Eel has been a mystery ever 

 since the days of Aristotle, though a small jjart of the story was- 

 made out even in ancient times. It was long ago ascertained 

 that the Eel, which seeks its food in rivers, descends to the sea 

 in autumn or early winter, and that it never spawns, nor even 

 becomes mature in fresh waters. The Eels which descend to- 

 the sea never return, but young eels or Elvers come up from the 

 sea in spring, millions at a time. The Elvers have been seen, 

 to travel along the bank of a river in a continuous band or eel- 

 rope, which has been known to glide upwards for fifteen days 

 together. It was, of course, concluded that spawning and early 

 development took place in the sea during the interval between, 

 the autumn and spring migration, but no certain information 

 came to hand till 1896. Meanwhile this gap in our knowledge 

 was a perplexity, almost a reproach to zoologists. The partially- 

 known migration of the Eel could not be harmonised with 

 the ordinary rule of migratory fishes. We tried to explairv 

 the passage of marine fishes into rivers at spawning time by the 

 supposition (a true supposition, as I think) that the river is less, 

 crowded than the shallow seas, and therefore a region in which 

 competition is less severe. The river is to some migratory fishes 

 what the tundras of Siberia are to some migratory birds, places 

 comparatively free from dangerous enemies, and therefore fit for 

 the rearing of the helpless young. But the Eel broke the lule, 

 and cast doubt upon the explanation. The Salmon, Sturgeoi> 

 and Lamprey feed and grow in the sea, and enter rivers to- 

 spawn. The Eel feeds and grows in rivers, but enters the sea 

 to spawn. What possible explanation could meet cases thus 

 diametrically opposite ? 



This was the state of matters when Grassi undertook to tell us 

 that part of the history of the Eel which is transacted in the 

 sea. When it leaves the river, it makes its way to very deep 

 water, and there undergoes a change. The eyes enlarge, and 

 become circular instead of elliptical ; the pectoral fins and the 

 border of the gill-cover turn black \. the reproductive organs, 

 only to be discovered by microscopic search before this time, 

 enlarge. The Eels, thus altered in appearance and structure, 

 lay their eggs in water of not less than 250 fathoms' depth. The 

 upper limit of the spawning-ground is nearly three times as far 

 from sea-level as the lOO-fathom line which we arbitrarily quote 

 as the. point at which the deep sea begins. The eggs, which are 

 large for a fish (27 mm. diam.), float but do not rise. The 

 young which issue from them are quite unlike the Eels of our 

 rivers ; they are tape-like, transparent, colourless, devoid of red 

 blood, and armed with peculiar teetk A number of different 

 kinds of such fishes had been previously known to the 

 naturalist as Leptocephali. Glinther had conjectured that they 

 were abnormal larvre, incapable of further development. Grassi 

 has, however, succeeded in proving that one of these Leptocephali 

 {L. brevirosiris) is simply a larval Eel ; others are larvae of 

 Congers and various Muraeuoid fishes. He has \\ iih infinite 



