^ 



HM 



otli 



ftf, 



^ n 



H „ •»■ 



. ■"ill 



^ 1 





IV 



r 



vinw 



..:8l ' 



fthe 



Vii 



1>U' V. 



11 Uit grail 



} 



.L::nas 



ind the .^ 



>4 W 



llicll ^: 



1 i. 



r 



badb^^^ 



^rar 



i-' 



lie c^'^ 



I 





Ch. XXXYII.] 



CONDITIONS OF EXISTENCE. 



319 



4 



of circuin stances to wliicli man can in a few tliousand years 

 subject any animal or plant under domestication. 



Were we to attempt to enumerate all the conditions which 

 Mr. Herbert Spencer has concisely termed the ' environment ' 

 of a species, they would be almost endless. They would com- 

 prise not only the mean temperature of the air or water, but 

 the extreme heat or cold in the different seasons of the year, 

 the quantity and intensity of sunshine at different periods, 

 the number of clear and of rainy days, the quantity of ice and 

 snow, the direction and strength of the wind, the pressure 

 of the atmosphere and its electrical state, the nature of the soil, 

 its elevation above the sea, the habits, instincts, and properties 

 of hundreds of contemporary animals and plants, some of them 

 friendly others inimical, the comparative abundance or rarity 

 of those species on which the food of a given animal or plant 

 may depend,— circumstances, many of them, wholly beyond 

 the control of the breeder or horticulturist. All of them 



mor 



emulate 



Dr. Hooker ascertained that the average rax.gu m verxicai 

 height of flowering plants in the Himalayan mountains 

 amounted to 4,000 feet, and the upper and lower limits 



of some 



much 



as 8,000 feet. If we transplant individuals which inhabit 

 the higher limits in these mountains into our British 

 gardens, we find that they are hardier, and better able to 

 stand the cooler climate of England, than those taken from 

 the inferior or warmer stations. This acclimatisation has 

 been the result of natural selection during thousands of gene 

 rations. The physiological constitution of the plant has 

 been acted upon, and a hardy race established, althouoh 

 the change may not have been sufficient to cause it to rank 



more 



sometimes be more 



m size than individuals of the same species living in the 



moist 



may 



slightly^ in the colour of its flowers, and, if deciduous, 

 the period of shedding its leaves 



in 



of growth. Yet its 



or in its general habits 



may 



on the whole 



sufficiently distinct to induce the botanist to rank 



it 



as 



fl . 



