HORIZONTAL DISTRIBUTION, GROWTH AND DYNAMICS OF DISPERSAL 285 



Numerous although our observations in the south have been, the major problem of periodic 

 coverage of so vast an area as the circumpolar sea could only it was found be resolved by an arbitrary 

 grouping of months regardless of the years in which they fell, it being manifestly impossible for a 

 single vessel, or even group of vessels, to have been everywhere in so large an area in every month of 

 a single year. Thus the distribution of the young stages, based on the data from the vertical nets, is 

 presented by two-monthly (in some instances three-monthly) periods, and the distribution of the 

 young, adolescent and adult stages, based on the data from the horizontal and oblique nets, by three- 

 monthly (seasonal) periods throughout the year. Attempts were in fact made at first to produce 

 monthly distributional charts, but the majority broke down owing to the size of the area under 

 investigation. It will be seen, however, that in the result the grouping I have used has provided a 

 measure of periodic coverage which at all material times permits certain broad inferences to be 

 drawn, both as to the facts of the distribution and its dynamics, inferences it seems, arbitrary though 

 the presentation of the data has been, that have substantial claim to reality. 



In marshalling the data from the vertical nets it appeared best, after careful consideration and much 

 experiment, to deal with the larval stages in groups rather than to produce separate bi-monthly or 

 tri-monthly charts for each individual larval stage. The eggs alone have been given charts to them- 

 selves. Thus the grouping of the eleven^ larval stages has been done as follows: group (i), the deep 

 living Second Nauplii, Metanauplii and First Calyptopes that in oceanic water^ occur between 

 1000 (or 1500) and 250 m., group (2), the shallow living First, Second and Third Calyptopes, 

 group (3), the shallow living First, Second and Third Furcilias and group (4), the shallow living 

 Fourth, Fifth and Sixth Furcilias, groups (2), (3) and (4) being confined for all practical purposes to 

 depths between 250 m. and the surface. Group (i) is a natural grouping in as much as it brings 

 together the total larval community rising from deep down to populate the surface waters above. 

 Moreover, it is a group confined exclusively to a single water mass, the warm deep current, and in 

 consequence every member of it can be assumed to be in a current with a southerly component except 

 (p. 123) below perhaps the greater part of the Weddell stream. Groups (2), (3) and (4) are also 

 natural groups for they too spend virtually their whole life in a single water mass, the Antarctic 

 surface layer, where again the horizontal movement is constant, in the East Wind drift to the west, in 

 the Weddell drift to the north and east. Furthermore, it so happens that in nature (pp. 225-8) 

 groups (2), (3) and (4) do not as a rule occur mixed indiscriminately in a single haul. The extent to 

 which they tend to be segregated or mingled can be judged by inspection of Figs. 34-7 and others on 

 later pages which show the developmental condition of the population in different seasons and places. 

 While there may be occasional intermingling it will be seen between two groups, one, however, always 

 preponderating over the other, in most instances where our nets have encountered them they have 

 encountered them singly, one stage or another in any one of the three as a rule being strongly dominant. 

 And so for this reason too groups (2), (3) and (4) may be said to be natural ones. 



The enormous accumulation of data gathered is presented on the following plan. Taking first the 

 material from the vertical nets and beginning with the eggs, the charts show in turn the distribution 

 and relative abundance of each larval stage,^ or group of larval stages, from the time when it first 

 appears in the plankton until the end of its natural span.* Thus, by two-monthly steps from November 



1 Including the First Nauplius there are in fact twelve. The earliest larval stage, however (p. 98), is so poorly represented 

 in our samples that its distribution, although it can be surmised, cannot be plotted. 



^ But not (p. 205) in shelf water. 



' For convenience the egg is regarded here as a larval stage, though strictly it is not. 



* In so far, however, only as that end can definitely be ascertained in regions accessible to vessels throughout the year. It 

 will be shown later that in the East Wind drift, which is closed to shipping for the greater part of the year, the larvae in all 

 phases of their surface development persist longer in the plankton than they do in lower ice-free latitudes. 



