396 



NA TURE 



{August 23, 1883 



of the bottom must be of the greatest consequence. The 

 colour of the botto.n does also, I believe, affect the 

 existence of certain species. I therefore recommend that 

 the nature of a bottom is not alone recorded, as, for 

 instance, thus, " clay bottom," " clay mixed with sand," 

 " stones with Algae," and " globigerina ooze," but that a 

 sample of the bottom is also in every case taken for 

 future chemical analysis. It should, however, be seen 

 that the sample is from the surface layer, and not from 

 those bilow, which may of course be of a quite different 

 nature. 



4. The Temperature and Saltness of the Water, as 

 well as its Chemical Composition near the Bottom. — 

 Particulars of these circumstances should always be 

 given exactly, as they do, no doubt, have a considerable 

 share in the production of a species. This is so evident 

 that it requires no further discussion. 



5. Tlie Period of the Research. — This is a point which 

 zoologists in most instances fail to record, and yet it 

 appears to me in several respects to be of great interest. 

 Everything in life is subjected to a gradual organic 

 change, and I believe that the fauna of the sea in this 

 respect does not differ. Those species of animals which 

 to-day appear within a certain locality are undoubtedly not 

 the same which were found there, say, a hundred years 

 ago, and still less the same as those which existed there 

 a thousand years ago, and what applies to the past ap- 

 plies with equal force to the future. The struggle for 

 existence causes the immigration of new forms, while 

 others must, so to say, make room for the newcomers and 

 thereby disappear. This lies in the progress of historical 

 development. For this reason it is necessary to state the 

 period of the research, and although science may have no 

 immediate gain from such dates, it will no doubt come 

 in course of time, and it is the duty of the student of 

 zoogeography to work as mu:h for posterity as for the 

 present. It is with these particulars as with those of 

 meteorological observations, viz. that one must possess 

 a number of observations, extending over a long period, 

 before the deductive result becomes of scientific value. 



But apart from the ultimate benefit which may be 

 derived in the future from these details, disputed per- 

 haps by some, the record of the time when the spe- 

 cimens are taken is of great importance to modern 

 science. It is thus well known that many of the 

 inhabitants of the sea, not only those which possess 

 perfect organs of locomotion, but also those which live a 

 somewhat stationary life, undertake, during certain 

 periods of the year, shorter or longer migratory wander- 

 ings. This is, however, as regards the lower Inverte- 

 brates, a circumstance which has been so little attended 

 to, that hardly any information exists on this point in 

 print. In connection with this peculiarity the records 

 which I advocate would be of great use. The causes of the 

 migratory movements may be very difficult to ascertain ; 

 but it is necessary in the first instance to demonstrate a 

 fact — the explanation will follow in course of time. I have 

 further indicated under (7) why I consider these state- 

 ments as to time of such importance. 



For the study of zoogeography in general it may be 

 sufficient alone to know what species occur within a 

 certain area, whether large or small. The student of 

 zoogeography compares those species which are to be 

 found either near or far from one another, he shows that 

 some of them are common to all those parts which he has 

 under consideration, that others belong to a few, and with 

 these data before him he attempts to discover the causes 

 of their appearance or absence in certain places. If 

 possible, he takes the most recent palseontological pheno- 

 mena into account too, he views the fauna of the present 

 day by the light of the past, and obtains thereby remark- 

 able and perhaps unexpected results. 



6. The Relative Plurality of the Individuals and the 

 Colonies. — It cannot, however, be denied that, should the 



mode of research indicated above be fruitful in some 

 respects, it will not give a complete account of the animal 

 life existing within a certain sphere. To obtain this it is 

 necessary to know the relative plurality of the individuals 

 and their colonies in every individual locality within the 

 sphere. On this point my opinion is that, in order 

 to understand correctly the composition of a fauna, it is 

 not enough to know those species which it embraces, the 

 zoologist must not be content with a mere enumeration 

 either with or without descriptions of the various species 

 and their distribution within a certain sphere, but he 

 must also take into consideration the relative plurality of 

 the individuals of each, i.e. he must, in other words, study 

 the statistics of the species. It is clear that zoogeography 

 must be based on these two propositions, as the science 

 would not advance far, if it should, for the comparison of 

 the fauna of two localities, rest on a mere enumeration of 

 the species occurring in such localities. A case might 

 certainly occur in which two localities could approximately 

 possess the same species, while their fauna were very 

 different in composition. To the student of zoogeography 

 this is no improbability. 



It cannot, of course, be demanded that the zoologist 

 occupied in dredging should immediately record the 

 number of every species the trawl brings up, as he must 

 for this purpose possess special qualifications, but science 

 is greatly benefited too by the course that, when the 

 various groups of species are distributed for research and 

 classification, the specialists in question in their works on 

 the same record exactly the number of individuals taken 

 of each species, and, with regard to animals forming 

 colonies, also how many colonies were found in each 

 place. When all the groups of the species had thus been 

 dealt with, the student specially interested could compare 

 the various species in every place investigated, and also 

 the individuals and colonies in each, and by such a com- 

 parison we should obtain a really complete knowledge of 

 the animal life in the locality investigated. If, however, no 

 notice be taken of the relative plurality of the individuals, 

 whether from want of study or attention to the import- 

 ance of the point, the picture which the reader of his work 

 obtains of the animal life in a certain locality will be very 

 vague and unsatisfactory indeed. 



7. The Relation between Males and Females in the 

 Same Place, and, if possible, at I 'arious Times and 

 ns. — The attention of zoologists should, in connec- 

 tion with the study of the relative plurality of individuals 

 and colonies, also be directed to this interesting circum- 

 stance, which of course is not related to zoogeography, 

 but. to biology. No doubt investigations of this relation 

 would lead to valuable discoveries. In some species, 

 with sexual difference, the males predominate, in others 

 the females, while in some they are evenly balanced. 

 Another point also of interest connected herewith is at 

 what period of the season the process of fertilisation takes 

 place, how long the pregnancy lasts, and when the females 

 cast their eggs ; whether these functions are confined to 

 certain seasons or not. Investigations of this point show 

 that in some cases these functions are regulated most 

 punctually, but in others not, and consequently it would 

 be a matter of great scientific interest to ascertain the 

 relations of the species on this point. For this reason it 

 is also of importance to state the exact time when the 

 examination of a certain locality took place. 



I consider that the points I have here discussed are the 

 principal ones for which the zoologist should, in order to 

 advance zoogeography, collect materials when making re- 

 searches on the fauna of the sea. They form in my opinion 

 the basis on which this science shall be founded for a higher 

 and more extended knowledge, and if the researches I 

 have here indicated are executed in a systematic manner 

 and with due care, my belief is that zoogeography will in 

 a short space of time reap excellent benefits and fruits 

 therefrom ; but here, as in every other branch of study, it 



