PLANKTONIC STUDIES. 639 
regards hunger as the cause of this, and the “tendency to explore a 
relatively great bulk of water.” In general, according to his view, 
“many larger pelagic animals bear the outspoken character of unfavor- 
able conditions of life, of a life of hunger.” 
Regarding the appearance of many pelagic animals in swarms, Hensen 
explains “that the young do not float, but swim freely. In consequence 
of this, the mother animals drive them away, and if the larve finally 
rise to the surface the former can not enter into competition with 
them” (22, p. 252), The accumulation of numbers of Physalia in great 
swarms stands, according to his view, in correlation with the mode ot 
movement. The animals which are capable of no independent move- 
ment of progression must remain rather closely crowded together, in 
order to be able to reproduce bisexually; those carried too far away 
must perish.” On the other hand it is to be noted that the Physalia is 
not, aS Hensen assumes, gonochoristic, but always hermaphroditic.* 
The above-mentioned phenomena, the similarity to water of the pe- 
Jagic fauna, the periodic appearance of many pelagic organisms in 
swarms, their abundant accumulation in the zodcurrents (p. 85), particu- 
larly their relation to the currents, are only a few of the greater prob- 
lems which planktology furnishes for human investigative energy. For 
‘these, as for so many other fields of biology, Charles Darwin, by the 
establishment of the descent theory, has opened to us the way to 
a knowledge of causes. We must study the complicated reciprocal 
relations of the organisms crowded together in the struggle for exist- 
ence, the interaction of heredity and variation, in order to learn to 
understand the life of the plankton. But in these plankton studies, as 
well in physiological as in morphological questions, we must use that 
method which Johannes Miiller, the discoverer of this field, always 
employed in a manner worthy of imitation: simultaneous “observation 
and reflection.” : 
* The cormi of all Physalide are moncecious, their cormidia monoclinic, Each 
single branch of the racemose gonodendron is monostylic, and bears one female and | 
several male medusoids. The facts were brought out thirty-five years ago by Huxley. 
(Compare my Report on the Siphonophorie: Zodlogy of the Challenger, vol. Xxv1u, 
pp. 347, 356.) 
