240 University of California Publications in Zoology [Vol. 18 



INTRODUCTION 



In working over the plankton collections made under the auspices 

 of the Scripps Institution certain peculiarities in the occurrence of 

 Salpa democratica, the smallest of the Salpae, led to an intensive study 

 of its distribution within the San Diego region. Although the study 

 is not yet complete, the relations revealed between fluctuations in sur- 

 face temperature and variations in surface distribution proved so strik- 

 ing and so significant, especially as regards the validity of the pre- 

 vailing plankton concept, as to make it advisable to publish at once 

 the results concerning this aspect of the problem. 



The way in which the morphological complexities in the life cycle 

 are reflected in the distributional data makes it necessary to describe 

 in some detail the successive stages in this cycle. Moreover, these 

 morphological implications of the distributional data afford indis- 

 putable evidence of the fundamental interdependence of morphological 

 and ecological research ; they demonstrate the necessity, if we are 

 ever rightly to interpret any biological phenomenon, of conducting 

 our investigations, not only in a rigorous and critical manner, but 

 also from the comprehensive natural history point of view so char- 

 acteristic of Darwin and his immediate followers — that point of view 

 which recognizes in all details of structure, function, behavior, and 

 variation the unifying fact of individual and species adaptation, and 

 which therefore holds all lines of research equally indispensable and 

 no fact of nature negligible. Again, the data uniquely demonstrate 

 that specificity in behavior is quite as far reaching as is specificity 

 in structure, the two generations of this species being quite as dis- 

 tinguishable from the way in which they are distributed as from the 

 way in which they are constructed. 



Lastly, the distributional data afford convincing evidence in sup- 

 port of the conclusion that S. democratica, a typical plankton species, 

 controls to a significant extent its own distribution, and that it d6es 

 so, in i^art at least, by means of locomotion. This is in direct contra- 

 diction to the prevailing plankton concept, according to which plank- 

 ton organisms are, as Johnstone (1908, p. 148) truly says, "particles 

 in the physical sense and behave as such." Of course, no sane 

 biologist actually believes this, but plankton is nevertheless defined 

 in nearly every book dealing with such organisms, as "all floating 

 organisms which are passively carried along by currents" (Murray 

 and Hjort, 1912, p. 309). This matter is considered at some length 

 at the close of this paper. 



