644 



ECOLOGY AND EVOLUTION 



gan to lose this resistance, however, and by 

 1929, 20 per cent of the acreage was lost. 

 New races of stem rust had appeared that 

 infected the formerly resistant wheat. An- 

 other resistant wheat variety known as 

 Ceres was first distributed in 1926. By 

 1933 it was planted in five milHon acres in 

 the United States and Canada, but in 1935 

 a physiologic race of stem rust (race 56) 

 attacked it and, with the help of favorable 

 weather, swept across the country in the 

 greatest rust destruction in history, kilHng 

 a fourth of the crop. Another rust-resistant 

 wheat called Thatcher began to be raised 

 in place of Ceres in 1934, and in 1941 it 

 occupied the acreages formerly devoted to 

 Ceres. A race of rust is known from South 

 America that attacks Thatcher wheat, but 

 by 1941 it had not yet invaded the fields 

 of central North America. 



New races of rust seem to arise soon 

 after the new races of wheat. In many cases 

 these new races are the result of hybridi- 

 zation of older races during the sexual fu- 

 sions on the barberry. Other physiologic 

 races may possibly be the result of muta- 

 tion. The genetic differences are not great 

 in either the rust or the wheat races. It is 

 obvious that this reciprocal evolution is the 

 result of the rise of slight genetic modifi- 

 cations against a background of the much 

 more complex adaptation of the rust spe- 

 cies to the wheat. We may thus see how 

 the preadapted rust may originate and 

 spread to physiologically and genetically 

 distinctive racial populations of its hosi 

 (Hart, 1944; Newton and Johnson, 1944; 

 Yu, Wang, and Fang, 1945). 



A most bizarre example of preadaptation 

 and postadaptation that includes an inter- 

 mixture of intraspecies group relations and 

 extreme environmental conditions is found 

 in the adjustments of the emperor penguin 

 {Aptenodytes forsteri. Fig. 219; see Mur- 

 phy, 1936). This bird, rarely found north 

 of the Antarctic circle, breeds at the edge 

 of pack ice in the vicinity of open water 

 containing its food supply. The adult is 

 able to shoot up out of the water, afight- 

 ing feet first upon ice IJ2 meters (about 5 

 feet) above the water surface. Unlike other 

 penguins of different genera (including the 

 Adelie penguin, Pygoscelis adeliae, the 

 only other bird nesting on the Antarctic 

 continent), the emperor has no available 

 nesting material to insulate the eggs from 



the cold ice. Its breeding season is during 

 the long Antarctic night. In adjusting to 

 these extreme conditions, this bird incu- 

 bates its egg by placing it on its feet be- 

 tween the belly and tail where a broad 

 transverse fold of skin covers the apex of 

 the egg. The period of incubation is be- 

 tween seven and eight weeks. When the 

 bird has a full stomach, the instinct to 

 brood is paramount; but digestion, excre- 

 tion, and hunger inhibit the brooding re- 

 action, and the egg, or young if it is 

 hatched, is turned over to another adult. 

 If an adult drops an egg, the egg is imme- 

 diately picked up by another bird. The 

 newly hatched chicks stay in the egg space 

 and take regurgitated food from the adult. 

 As they grow larger, they sit outside still 

 on the feet of the adult, and later they 

 squat on the ice and put the head in the 

 "warming-chamber." 



It has been observed that from one in 

 five to one in twelve of a total colony popu- 

 lation possessed an egg. Colonies have 

 been estimated to contain 7500 birds, in- 

 cluding young. Both sexes incubate the egg, 

 and both have a bare patch of skin on the 

 lower abdomen that is used in incubation. 

 Only a few birds breed during a single sea- 

 son, but the unmated individuals show as 

 much parental solicitude for the egg and 

 chick as do the actual parents, thus allow- 

 ing time necessary for procuring food. So 

 strong is the instinct for egg and chick care 

 that addled eggs, dead chicks, and even 

 lumps of ice of the right size are tucked 

 on the feet and covered with the feathery 

 muff. There is so much competition for the 

 freed young that they are often handled 

 roughly by the adults and may even be- 

 come lost or frozen during the argument. 

 In one rookery it was estimated that the 

 mortality among the young was 77 per 

 cent. The average individual age is thirty- 

 four years. The young take about one and 

 one-half years to gain the mature coat. The 

 downy stage lasts only four to five months, 

 in contrast with twice this length of time 

 in the king penguin. 



Its close relative, the king penguin {Ap- 

 tenodytes patagoniciis) , lives in regions 

 with air temperatures usually above freez- 

 ing and rarely if ever dropping below 

 — 18° C. The emperor penguin lives and 

 breeds in regions with air temperatures 

 from -18° C. to -62° C. (-80° F.). The 



