1044 LIFE : OUTLINES OF GENERAL BIOLOGY 



been absolutely dissipated. I am aware of no definite permanent 

 change in any introduced species." But inquiry must be made in 

 other fields to see whether there is nothing to be said for the old 

 view. 



(5) So many naturalisations have been effected in different parts 

 of the world that one would expect to find it easy to collect instances 

 of change of habit; but there seems to be a strong conservative 

 tendency among animals introduced into new haunts. When 

 domesticated animals are transported to a wilder country there is 

 sometimes an interesting individual rehabilitation of a long lost 

 ancestral trait. Thus cows taken from Scotland to wilder conditions 

 in California have been known to hide their calves in the thicket 

 when they went to graze in the open — a revival of an ancient 

 custom among wild cattle. Sometimes, however, something novel 

 occurs. Thus, though sheep have been established in New Zealand 

 for only about a hundred years, they have shown in some places 

 the novel habit of stripping off long pieces of bark from the gaya 

 trees. On the other hand, the Australian dingo, almost certainly a 

 somewhat primitive type of domestic dog, feral for many centuries, 

 remains with characteristic dog-like habits, except that it does not 

 bark. On the whole it seems that change of habit in consequence 

 of naturalisation is infrequent and very gradual. 



(6) Some types are quickly at home in a new country, but do 

 not show any external change. Horses, rabbits, rats, sparrows, and 

 fowls are usually somewhat indifferent to change of climate, while 

 a tough animal like the Yak of the Tibetan mountains refuses to 

 thrive below a certain altitude, and it is said that Newfoundland 

 dogs will not live in India. Very blonde people usually find the 

 Tropics difficult, yet some men thrive anywhere. 



Individual adjustments may be of much importance though they 

 do not meet the eye; as in the case of adaptation to high altitudes. 

 When a man accustomed to life near sea-level settles in a place like 

 Johannesburg, with an altitude of about 6,000 feet, there is likely 

 to be, if he thrives, an increase in the haemoglobin content of the 

 blood. This is due at first to a reduction in the volume of the plasma 

 and a concentration of corpuscles, but secondarily to an increased 

 corpuscle-formation. Increased activity has been observed in the 

 red bone marrow, which makes corpuscles. At very high altitudes 

 there is some evidence of an increase in the affinity of haemoglobin 

 for oxygen. Another change is demonstrable in the reaction of the 

 blood, to the acid side, and this is also useful since less CO2 is 

 required to stimulate the respiratory centre, thus increasing pul- 

 monary ventilation. According to some authorities there is also a 

 change in the epithelium lining the cavity of the lungs, which 

 becomes able to force oxygen from the alveolar air into the blood. 

 We have referred to this adaptation to high altitudes, which is in 



