August 26 1897] 



NATURE 



403 



locally common. Let us be slow to enter into controversies. 

 After they have been holly pursued for some time, it generally 

 turns out that the disputants have been using words in dlflerent 

 senses. Discussion is excellent, controversy usually barren. 

 Vet not always ; the Darwinian controversy was heated, and 

 nevertheless eminently productive ; all turns upon the temper 

 of the men concerned, and the solidity of the question at issue. 

 One more hint to young students. Perhaps no one ever carried 

 through a serious bit of work without in some stage or other 

 longing to drop it. There comes a time when the first impulse 

 is spent, and difficulties appear which escaped notice at first. 

 Then most men lose hope. That is the time to show that we 

 a^-e a little better than most men. I remember as a young man 

 <irawing much comfort from the advice of a colleague, now an 

 eminent chemist, to whom I had explained my difficulties and 

 fears. All that he said was: " Keep at it," and I found that 

 nothing more was wanted. 



I greatly believe in the value of association. It is good that 

 two men should look at every doubtful structure and criticise 

 ■every interpretation. It is often good that two talents should 

 «nter into partnership, such as a talent for description and a 

 talent for drawing. It is often good that an experienced in- 

 vestigator should choose the subject and direct the course of 

 work, and that he should be helped by a junior, who can work, 

 but cannot guide. It seems to me that friendly criticism before 

 publication is often a means of preventing avoidable mistakes. I 

 am sorry that there should be any kind of prejudice against co- 

 operation, or that it should be taken to be a sign of weakness. 

 There are, I belie%'e, very few men who are so strong as not to 

 be the belter for help. One difficulty would be removed if 

 known authors were more generous in acknowledging the help 

 -of their assistants. They ought not to be slow to admit a real 

 helper to such honour as there may be in joint-authorship. 



Among the most important helps to the student of life- 

 histories must be mentioned the zoological stations now main- 

 tained by most of the great nations. The parent of all these, 

 •the great zoological station at Naples, celebrated its twenty-fifth 

 anniversary last April, so that the whole movement belongs to 

 our own generation. How would Spallanzani and Vaughan 

 Thompson and Johannes Miiller have rejoiced to see such 

 facilities for the close investigation of the animal life of the sea I 

 The English-speaking nations have taken their fair share of the 

 splendid work done at Naples, and it is pleasant to remember 

 that Darwin subscribed to the first fund, while the British 

 Association, the University of Cambridge and the Smithsonian 

 Institution have maintained their own tables at the station.' 

 The material support thus given is small when compared with 

 the subsidies of the German Government, and not worth men- 

 tion beside the heroic sacrifices of the Director, Dr. Anton 

 Dohrn, but as proofs of lively interest in a purely scientific 

 ■enterprise they have their value. Marine stations have now 

 multiplied to such a point that a bare enumeration of them 

 would be tedious. Fresh-water biological stations are also 

 growing in number. Forel set an e.xcellent example by his 

 investigation of the physical and biological phenomena of the 

 Lake of Geneva. Dr. Anton Fritsch of Prag followed with his 

 movable station. There is a well-equipped station at Plon 

 •among the lakes of Holstein, and a small one on the Muggelsee 

 near Berlin. The active station of Illinois is known to me only 

 by the excellent publications which it has begun to issue. 

 France, Switzerland, Sweden and Finland all have their fresh- 

 water biological stations, and I hope that England will not long 

 jremain indifferent to so promising a sphere of investigation. 



Biological work may answer many useful purposes. It may 

 be helpful to industry and public health. Of late years the 

 entomologist has risen into sudden importance by the vigorous 

 steps taken to discourage injurious insects. I have even known 

 a zoological expert summoned before a court of law in order to 

 say whether or not a sword-fish can sink a ship. I would not 

 on any account run down the practical applications of Biology, 

 but I believe that the first duty of the biologist is to make science, 

 and that science is made by putting and answering questions. 

 We are too easily drawn off from this, which is our main busi- 

 ;ness, by self-imposed occupations, of which we can often say 

 iiothing better than that they do no harm except to the man who 

 undertakes them. There are, for example, a good many lists 

 of species which are compiled without any clear scientific 

 object. We have a better prospect of working to good purpose 

 when we try to answer definite questions. I propose to spend 

 1 To this list may oow be added the University of Oxford. 



NO. 1452, VOL. 56] 



what time remains in putting and answering as well as I can a 

 few of the questions which occur to any naturalist who occupies 

 himself with life-histories. Even a partial answer — even a 

 mistaken answer is better than the blank indifference of thp 

 collector, who records and records, but never thinks about his 

 facts. 



The first question that I will put is this :— Why do some 

 animals undergo transformation while others do not ? It has long 

 been noticed ' that as a rule fresh- water and terrestial animals do 

 not go through transformation, while their marine allies do. Let 

 us take half a dozen examples of each : — 



Fluviatile or terrestrial. 

 Without tr.-insformation. 



Crayfish. 



Earthworm. 



Helix. 



Cyclas. 



Hydra. 



&c. 



Marine. 

 With transformation. 

 Crab. 



Polygordius. 

 Doris, l^iolis. 

 Oyster. 



Most Hydrozoa. 

 &c. 



We get a glimmer of light upon this characteristic difference 

 when we remark that in fresh-water and terrestrial species the 

 eggs are often larger than in the allied marine forms. A large egg 

 favours embryonic as opposed to larval development. An embryo 

 which is formed within a large egg may feed long upon the food 

 laid up for it, and continue its development to a late stage before 

 hatching. But if there is little or no yolk in the egg, the 

 embryo will turn out early to shift for itself. It will be born as 

 a larva, provided with provisional organs suited to its small size 

 and weakness. Large eggs are naturally fewer than small ones. 

 Does the size depend on the number, or the number on the size? 

 To answer in a word, I believe that the size generally depends 

 on the number, and that the number is mainly determined by the 

 risks to which the species are exposed. At least so many eggs 

 will in general be produced as can maintain the numbers of the 

 species in spite of losses, and there is some reason to believe that 

 in fresh waters the risks are less than in the shallow seas or at 

 the surface of the ocean.'- In most parts of the world the fresh 

 waters are of small size, and much cut up. Every river-basin forms 

 a separate territory. Isolation, like every other kind of artificial 

 restriction, discourages competition, and impedes the spread of 

 successful competitors. In the shallow seas or at the surface of 

 the ocean conquering forms have a free course ; in lakes and 

 rivers they are soon checked by physical barriers. 



A large proportion of animals are armour-clad, and move about 

 with some difficulty when they ha%-e attained their full size. The 

 dispersal of the species is therefore in these cases effected by 

 small and active larvae. Marine animals (whether littoral or 

 pelagic) commonly produce vast numbers of locomotive larvie, 

 which easily travel to a distance. Floating is easy, and swim- 

 ming not very difficult. A very slightly built and immature 

 larva can move about by cilia, or take advantage of currents, 

 and a numerous brood may be dispersed far and wide while they 

 are mere hollow sacs, without mouth, nerves or sense-organs. 

 Afterwards they will settle down, and begin to feed. In fresh 

 waters armour is as common, for all that I know, as in the sea, 

 but locomotive larvce are rare.* There is no space for effective 

 migration. Even a heavy-armoured and slow-moving crustacean 

 or pond-snail can cross a river or lake, and to save days or hours is 

 unimportant. In rivers, as Sollas has pointed out, free-swim- 

 ming larvce would be subject to a special risk, that of being swept 

 out to sea. This circumstance may have been influential, but 

 the diminished motive for migration is probably more important. 

 At least an occasional transport to a new area is indispensable 

 to most fresh-water organisms, and very unexpected modes of 



1 Darwin, " Origin of Species," chap. xiii. ; Fritz MuUer, " Fur Darwin," 

 chap. vii. 



2 Indications are given by the survival in fresh waters of declining groups, 

 e.g. Ganoid Fishes, which, when dominant, maintained themselves in the 

 sea ; and by the not uncommon case of marine animals which enter rivers to 

 spawn. I do not attempt to count among these indications the supposed 

 geological antiquity of fluviatile as compared with marine animals. Some 

 marine genera are extremely ancient (Lingula, Nucula, Trigonta, Nautilus); 

 a perfectly fair comparison is almost impossible ; and great persistence does 

 not necessarily imply freedom from risks. In the M.iUusca, which afford a 

 good opportunity of testing the effect of habitat uoon the number of the eggs, 

 marine species seem to produce more eggs as a rule than fluviatile, and these 

 many more than terrestrial species. 



3 Dreyssensia and Cordylophora are examples of animals which seem to 

 have quite recently become adapted to fresh-water life, and have not yet lost 

 their locomotive larvae. Many instances could be quoted of marine forms 

 which have become fluviatile. The converse is, I believe, comparatively 



