August 26, 1897] 



NATURE 



405 



pains compared a number of Leptocephali, and coordinated 

 their stages, making out some particularly important ones by 

 the direct observation of live specimens. 



You will not unnaturally ask how Grassi or anybody else can 

 tell what goes on in the sea at a depth of over 250 fathoms. 

 His inquiries were carried on at Messina, where the local circum- 

 stances are very fortunate. Strong currents now and then boil 

 <up in the narrosv strait, sweeping to the surface eggs, larvre, and 

 a multitude of other objects which at ordinary seasons lie un- 

 disturbed in the tranquil depths. Further information has been 

 got by dredging, and also by opening the body of a sun-fish 

 {Ortha^oriscus mo/a), which at certain times of the year is taken 

 at the surface, and is always found to contain a number of Lepto- 

 cephali. When a Leptocephalus has completed its first stage of 

 growth, it ceases to feed, loses bulk, and develops pigment on 

 the surface of the body^ At the same time the larval teeth are 

 cast, and the larval skeleton is replaced. Then the fish begins 

 to feed again, comes to the surface, enters the mouth of a river, 

 and, if caught, is immediately recognised as an Elver or young 

 Eel. It is now a year old, and about two inches long. 



This history suggests a question. Are the depths of the sea free 

 from severe competition ? The darkness, which must be nearly 

 or altogether complete, excludes more than the bare possibility 

 of vegetation. A scanty subsistence for animals is provided by 

 the slowly-decomposing remains of surface-life. When the 

 dredge is sunk so low, which does not often happen, it may 

 bring up now and then a peculiar and specially modified in- 

 habitant of the dark and silent abyss. There cannot, we should 

 think, be more than the feeblest competition where living things 

 are so few, and the mode of life so restricted. (Joing a step 

 further, we might predict that deep-sea animals would lay few 

 eggs at a time, and that these would develop directly — i.e. with- 

 out transformation. The risk of general reasoning about the 

 affairs of living things is so great that we shall hold our conjec- 

 tures cheap unless they are confirmed by positive evidence. 

 Happily this can be supplied. The voyage of the Challenger 

 has yielded proof that the number of species diminishes with 

 increasing depth, and that below 300 faihoms living things are 

 few indeed.' Dr. John Murray gives us the result of careful 

 elaboration of all the facts now accessible, and tells us that the 

 majority of the abyssal species develop directly (Nature, 

 March 25, 1897). 



We seem, therefore, to have some ground for believing that 

 the depths of the sea resemble the fresh waters in being com- 

 paratively free from enemies dangerous to larvs. The Eel finds 

 a safe nursery in the depths, and visits them for the same reason 

 that leads some other fishes to enter rivers. It may be that the 

 •depths of the sea are safer than rivers, in something like the 

 same degree and for the same reasons that rivers are safer than 

 shallow seas. But we must be careful not to go too fast. It 

 may turn out that deep recesses in the shallower seas— holes of 

 limited extent in the sea-bottom — enjoy an immunity from 

 dangerous enemies not shared by the great and continuous 

 ocean-floor.^ 



After this short review of the facts I come to the conclusion 

 that the general rule which connects the presence or absence of 

 transformation with habitat is well-founded, but that it is apt to 

 be modified and even reversed by highly special circumstances. 

 The effect of habitat may, for instance, be over-ruled by para- 

 sitism, parental care, a high degree of organisation, or even by 

 a particular trick in egg-laying. The direct action of the 

 medium is probably of little consequence. Thus the difference 

 between fresh and salt water is chiefly important because it pre- 

 vents most species from passing suddenly from one to the other. 

 But the abyssal and the fluviatile faunas have much in common, 

 as also have the littoral and the pelagic faunas. Relative density 

 and continuity of population seem to be of vital importance, and 

 it is chiefly these that act upon the life-history. 



In Zoology, as in History, Biography, and many other 

 studies, the most interesting part of the work is only to be 

 enjoyed by those who look into the details. To learn merely 

 from text-books is notoriously dull. The text-book has its uses, 

 but, like other digests and abridgments, it can never inspire 

 enthusiasm. It is the same with most lectures. Suppose that 

 the subject is that well-worn topic, the Alternation of Genera- 



1 Challenger Reports. ".Summary of Scientific Results" (1895), pp. 

 1430-96. 



'- 1 am aware that other things affect the interests of animals, and in- 

 directly determine their structure, besides danger from living enemies. So 

 complicated a subject can only be discussed in a short space if large omis- 

 -sions are tolerated. 



NO. 1452, VOL. 56] 



tions. The name recalls to many of us some class-room of our 

 youth, the crudely coloured pictures of unlikely animals which 

 hung on the walls, and the dispirited class, trying to write down 

 from the lecture the irreducible minimum which passes a candi- 

 date. The lecturer defines his terms and quotes his examples ; 

 we have Salpa, and Aurelia, and the Fern, and as many more as 

 time allows. How can he expect to interest anybody in a 

 featureless narrative, which gives no fact with its natural circum- 

 stances, but mashes the whole into pemmican ? What student 

 goes away with the thought that it would be good and pleasant 

 to add to the heap of known facts ? The heap seems needlessly 

 big already. And yet every item in that dull mass was once 

 deeply interesting, moving all naturalists and many who were 

 not naturalists to wonder and delight. The Alternation of 

 Generations worked upon men's minds in its day like Swam- 

 merdam's discovery of the butterfly within the caterpillar, or 

 Trembley's discovery of the budding Hydra, which when cut 

 in two made two new animals, or Bonnet's discovery that an 

 Aphis could bring forth living young without having ever met 

 another individual of its own species. All these wonders of 

 nature have now been condensed into glue. But we can at any 

 time rouse in the minds of our students some little of the old 

 interest, if we will only tell the tale as it was told for the first 

 time 



Adalbert Chamisso, who was in his time court-page, soldier, 

 painter, traveller, poet, novelist, and botanist, was the son of 

 a French nobleman. When he was nine years old, he and all 

 the rest of the family were driven out of France by the French 

 Revolution. C'lamisso was educated anyhow, and tried many 

 occupations before he settled down to Botany and light literature. 

 In 181 5 he embarked with Eschscholtz on the Russian voyage 

 round the world commanded by Kotzebue. The two naturalists 

 (for Chamisso is careful to associate Eschscholtz with himself, 

 and even to give him priority) discovered a highly curious fact 

 concerning the Salpre, gelatinous Tunicates which swim at the 

 surface of the sea, sometimes in countless numbers. There are 

 two forms in the same species, which differ in anatomical struc- 

 ture, but especially in this, that one is solitary, the other com- 

 posite, consisting of many animals united into a chain which 

 may be yards long. Chamisso and Eschscholtz ascertained that 

 the solitary form produces the chain-form by internal budding, 

 while the chain-form is made up of hermaphrodite animals 

 which reproduce by fertilised eggs.' There is thus, to use 

 Chamisso's own words, " an alternation of generations. ... It 

 is as if a caterpillar brought forth a butterfly, and then the 

 butterfly a caterpillar." Here the phrase bring forth is applied 

 to two very different processes, viz. sexual reproduction and 

 budding. Chamisso's phrase, " alternation of generations," is 

 not exact. Huxley would substitute alternation of geturation 

 with gemmation, and if for shortness we use the old term, it 

 must be with this new meaning. Subsequent investigation, 

 besides adding many anatomical details, has confirmed one 

 interesting particular in Chamisso's account, viz that the embryo 

 of Salpa is nourished by a vascular placenta '^ The same voyage 

 yielded also the discovery of Appendicularia, a permanent 

 Tunicate tadpole, and the first tadpole found in any Tunicate. 



Some ten years after the publication of Chamisso's alternation 

 of generations in Salpa, a second example was found in a 

 common jelly-fish (Aurelia). Not a few Hydrozoa had by this 

 time been named, and shortly characterised. Some were polyps, 

 resembling the Hydra of our ponds, but usually united into per- 

 manent colonies ; others were medusae, bell-shaped animals 

 which swim free in the upper waters of the sea. It was already 

 suspected that both polyps and medusoe had a common struc- 

 tural plan, and more than one naturalist had come very near 

 to knowing that medusii; may be the sexual individuals of 

 polyp-colonies. 



This was the state of matters when an undergraduate in 

 Theology of the University of Chrisliania, named Michael Sars. 

 discovered and described two new polyps, to which he gave the 

 names, now familiar to every zoologist, of Scyphistoma and 

 Strobila. In the following year (1830) Sars settled at Kinn, 

 near Bergen, as parish priest, and betook himself to the life-long 

 study of the animals of the Norwegian seas. He soon found out 

 that his Scyphistoma was merely an earlier stage of his Strobila. 

 Scyphistoma has a Hydra-like body, less than half an inch long, 



1 Brooks maintains that the solitary Salpa, which is female, produces a 

 chain of males by budding, and lays an egg in each. These eggs are 

 fertilised while the chain is still immature, and develop into females (solitary 

 Salpae). The truth of this account must be determined by specialists. 



2 Cuvier had previously noted the fact. 



