638 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
the conviction that “the more accurately the investigation has been 
made, so much the 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 investigations are made, the 
greater the number of individuals of a species compared, the intenser the 
study of individual variation, by so much more impossible becomes tie 
distinction of actual species, so much more arbitrary the subjective limits 
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 biology. 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 Ohallenger 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 German National 
expedition, the first great German undertaking in the field, may 
promote many planktonic 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 obtain 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 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 
point of view is to blame. Thus, for example, I have attempted to 
explain the remarkable ‘similarity to water of the pelagic fauna,” the 
transparency of the colorless glassy animals, in 1866, in my General 
Morphology (11, 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 has briefly, in his careful “Investigations upon the Stickleback,” 
given expression to the same conviction in the following words: ‘ All the conelu- 
sions here deduced by me are simply and solely founded 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 
confirm 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 great number of individuals from 
different localities accurately and methodically. He will then soon see that proofs of 
the theory of descent by this means are found in great numbers at all times, if only one 
does 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 and 
unbiased systematist. 
