G38 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



the conviction that ■•tlic more ;H-ciiratoly tho investigation has beeu 

 made, so ninrli tlic iiioic plain becomes tlie distinction of species" (9, 

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

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

 clusion: "T//6' more accurntcly the systematic rnvestiffatio7i,s are made, the 

 greater the nionber of i)i<lirichtals of a speeies compared^ the intenser the 

 study of iiidiridiial variation, by so much more impossible becomes the 

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

 of their extent, so much stronf/cr the conviction of the truth of the Theory 

 of Descent r* 



TLANKTOLOCilCAL PROBLEMS. 



The wonderful world of organic life, which tills the vast oceans, ofit'ers 

 a fund of very interesting sul)jects. Without ({uestion, it is one of the 

 most attractive and profitable fields of biology. If we consider that 

 the greater part of this field has been open to ns 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 upon a brilliant 

 future for planktology. 



Above all we ought to cherish the hope that our Cierman Natio7ial 

 expedition, the first great Gerin"an undertaking in the field, may 

 promote many planktonic problems, and that the six naturalists who, 

 undcr.such favorable conditions and with such important instruments, 

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

 the net were able to obtain a ricli 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 upon the matter now. Among 

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

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

 l)oiiit of view is to blame. Thus, for exam])le, I have attem]»ted to 

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

 transparency of the colorless glassy animals, in 18G(), in my General 

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

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



* F. Hcincke has briefly, in his careful "Investigations upon the Stickleback," 

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

 sions here deduced by nio are simply and sfdely founcbd upon the comparison of 

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

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

 couliriii 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 nnother, must begin by compaiing a very f/rcat inuiihvr of iiidiriditulu from 

 different localitiiH accurately and nu^thodically. He will then soon see that proofs of 

 the theonj of d'cscrnt hij this meanit are found in (jrent numbers at all times, if only one 

 does not span- the jtains to trac«> thrm out." (Ofvcrsigt af K. V. Akad. Forh. Stock- 

 holm, lb8'J, No. (), p. 110.) This view of Heincke is shared by every experienced and 

 unbiased systematist. 



