G38 EEPORT OP COMMISSIONER OF FISH AND FISHERIES. 



the conviction tluit '' the more accurately tlie investigation lias been 

 made, so much tlie more plain becomes the distinction of species" (9, 

 p. 100), On the other side I, like Charles Darwin, through many years 

 of comparative and systematic work, have arrived at the opposite con- 

 clusion: " The more accurately the systematic incestUjations are made, the 

 greater the number of individ^tals of a S2)fcies compared^ the intenser the 

 study of individual variation, hy so much more impossible becomes the 

 distinction of actual species, so much more arbitrary the subjective Uuiits 

 of their extent, so much stronger the conviction of the truth of the Theory 

 of Descent.'^* 



PLANKTOLOGICAL PROBLEMS. 



The wonderful world of organic life, which fills the vast oceans, offers 

 a fund of very interesting subjects. Without question, it is one of the 

 most attractive and profitable fields of bioh)gy. If we consider that 

 the greater part of this field has been open to us scarcely fifty years, 

 and if we wonder at the new discoveries which the Challenger expedition 

 alone has brought to light, then we ought to count iipon a brilliant 

 future for planktology. 



Above all we ought to cherish the hope that our German National 

 expedition, the first great German undertaking in the field, may 

 promote many x>lanktonic problems, and that the six naturalists who, 

 under such favorable conditions and with such important instruments, 

 studied the oceanic plankton for ninety-three days and in 400 hauls of 

 the net were able to ol)tain a rich collection of pelagic organisms, will 

 by their careful working up of these enrich our knowledge many fold. 

 However, the preliminary contributions of Hensen (22) and Brandt (23) 

 give us no means of passing judgment ni^on the matter now. Among 

 the results which the former has briefly given to the Bejlin Academy 

 few require consideration; but for this the difference of our general 

 ])oint of view is to blame. Thus, for exami)le, I have attempted to 

 explain the remarkable "similarity to water of the pelagic fauna," the 

 transparency of the colorless glassy animals, in 1860, in iny General 

 Morphology (ii, p. 242), according to Darwin's Theory of Selection, by 

 natural selection of like colors (30, p. 248). Hensen, on the other hand, 



* F. Heincke lias briefly, in his careful "Investigations npon' the Stickleback," 

 given expression to the same conviction in the following words : " All the conclu- 

 sions here deduced by me are simply and solely I'ounded upon the comparison of 

 very many individuals of living species, or, in other words, upon the study of indi- 

 vidual variation. I am convinced that in essentials the study of embryology will 

 conlirm my theory. It will be a proof of this, that he who wishes accurately to 

 describe related species, and races of a species, and to study their genealogical rela- 

 tion to one another, must begin by comparing a very {ircat number of ifidividuals from 

 different, localities accurately and methodically. He will then soon see thai proofs of 

 the theory of descent by tliis means are found in great nmnbers at all times, if only one 

 docs not spare the pains to trace them out." (Ofversigt af K. V. Akad. Forh. Stock- 

 holm, 1889, No. 6, p. 410.) This view of Heincke is shared by every experienced ami 

 unbiased svetcmatist. 



