THE FEEDING MIGRATIONS OF THE BALEEN WHALES 429 



In SO far as the southern stocks are concerned it might be suggested that during a much colder 

 regime, associated with a northerly extension of the polar ice-cap, the cold Antarctic currents 

 carrying the krill penetrated into lower latitudes than they do today, perhaps so far north that at one 

 time both breeding places and feeding places were coincident, or fell not very far apart. It could have 

 happened then that when the ice began to retreat, and the northern limit of euphausian abundance 

 to go with it, the whales learnt to follow their food into the higher and higher latitudes towards which 

 it would imperceptibly have been receding. There is evidence from bottom cores (Deacon, 19606) 

 that the Antarctic convergence has in fact retreated, to where we now find it, from a more northerly 

 position held during the Glacial Period, and that during this same epoch the influence of the Antarctic 

 bottom water must have been more far-reaching than it is today. 



Similar climatological change, involving a northward shift of feeding stuff from low to high lati- 

 tudes, could also be invoked to explain the extensive migrations of the northern whales. 



Turning again to Antarctica and its geological history we find in this something that might have 

 been contributing to the evolution of the feeding migration independently of climatological change. 

 There must have been a time I imagine (see Matthews, 1959) when the scattered arcuate group of 

 islands now known as the Scotia Arc was an unbroken mountain chain linking the Andes with 

 Graham Land. Such a barrier, presenting a buffer to the Weddell stream, as it swept up from the 

 south, might it is possible have diverted this strong current far into what are now temperate and 

 subtropical latitudes, carrying the krill into, or very close to, the very regions where the whales 

 now winter, pair and give birth to their calves. It could be imagined then that the gradual disruption 

 or submergence of this mountain chain brought in its train a gradual weakening of the Weddell 

 stream, a slow southerly shift of the krill it carried and again a movement of the whales that over the 

 ages involved them in gradually but imperceptibly increasing southward journeys as they followed 

 their retreating food. 



Perhaps in Antarctica geological and climatological history are equally linked up with the evolution 

 of this remarkable rhythm. 



REVIEW OF DYNAMICS OF DISTRIBUTIONAL CONTROL 



It has been shown repeatedly in this report that the warm deep current carrying the ascending 

 Second Nauplii, Metanauplii and younger First Calyptopes must play an important part in main- 

 taining the krill population within its normal geographical limits and that in the Atlantic sector, 

 where, underneath the Weddell stream, the cold bottom water is presumed to be carrying hatching eggs 

 to the north and east (see pp. 100-102), there seems to be a strong likelihood that the southward 

 movement of the warm layer, through which the resultant larvae are rising, will by some route 

 lead to a replenishment of the coastal population in the East Wind drift that is moving clockwise 

 round the Weddell Sea (see pp. 122-3). It <^^'^> however, be in the Atlantic sector alone that the 

 high latitude East Wind population is being replenished by larval incursions from so far north, for 

 elsewhere, throughout the vast extent of the circumpolar West Wind drift east about from meridian 

 30° E, there is, it appears (p. 194, Figs. 24 and 25), neither a northerly population of spent or 

 gravid females nor a northerly population of deep south-borne larvae (p. 200, Fig. 28 and p. 307, 

 Fig. 72) from which such replenishment could spring. It has been shown too (p. 169, Table 35), 

 that throughout their circumpolar range the older stages of this species in the staple or over 20 mm. 

 class do not at any time of the year, and least of all (p. 158, Table 28) in the West Wind drift, undergo a 

 mass descent from the north-going Antarctic surface layer into the warm counter-flowing deep current 

 below, so that again the west-going population in the East Wind drift cannot, anywhere it seems, not even 



