1042 LIFE : OUTLINES OF GENERAL BIOLOGY 



sheep taken to a colder area have offspring with thicker fleece, 

 or when plants taken to high altitudes have generations of progeny 

 with increasingly wooUy leaves, lessening the loss of water. But the 

 term may refer to becoming adapted to a change of climate within 

 the same country, as might occur if an Ice Age or a period of aridity 

 set in. The difficulty, as will become evident, is to get at facts which 

 demonstrate acclimatisation as an actual process, if the term is 

 used in the strict sense of becoming in the course of generations 

 adaptively changed so as to withstand climatic influences which 

 were at first unfavourable. When a plant or animal is transported 

 to a new country and gets on well, in itself and in its descendants, 

 it is said to be naturalised. But there may be naturalisation with- 

 out demonstrable acclimatisation. Thus some investigators, e.g. 

 G. M. Thomson (1922), who find little evidence of acclimatisation 

 in the strict sense, prefer to keep to the term naturalisation, which 

 expresses an indisputable fact that organisms may thrive well 

 when taken to another and in some aspects different country. 

 Willis (1922) defines acclimatisation as "the accustoming of plants 

 to new conditions and climates tiU they are not only capable of 

 growing there, but also of reproducing themselves freely". Thus, 

 though the cherry and apple wiU grow well in the hills of Ceylon, 

 they are not acclimatised, for they do not produce fertile seeds 

 Effects of Naturalisation (1922, p. 29). 



When plants and animals get a footing in a new and different 

 country, what changes may be looked for? (i) There is often aj 

 marked increase in the number of individuals in a given area, as » 

 is familiarly illustrated by the multiplication of the rabbits in 

 Australia, or of greenfinches and skylarks in New Zealand. The 

 reason is twofold: favourable conditions, such as abundant food, 

 may increase the rate of multiplication, and there may be an 

 absence of the enemies and other checks which kept the numbers 

 down in the old country. There have been some costly verifications 

 of the numerical increase that is apt to follow naturalisation, as 

 in the familiar case of rabbits in Australia and New Zealand. 



(2) If the introduced plants or animals do not multiply at aU, 

 there obviously cannot be any naturalisation, but it is possible that 

 the reproductivity may be diminished without making naturalisa- 

 tion impossible. Thus the Canadian Pondweed has taken firm hold 

 in European streams and canals, although it has ceased to multiply 

 sexually; and as above mentioned some fruit trees may flourish 

 well in new countries, yet never produce fertile seeds. 



(3) Another consequence that has been repeatedly noted is 

 increase in individual size and apparent vigour. The new conditions 

 prove unusually stimulating. Speaking of plants introduced into 

 New Zealand, G. M. Thomson writes (1922, p. 504) : "Water-cress — 

 a plant of two to four feet in length in European waters — ^grew in 



