CHAPTEE X. 



THE EFFECTS OF CONDITIONS OF LIFE IN 

 GENERAL. 



Local conditions of life perhaps the cause of local races, but proof 

 of this is as a rule impossible American and European trees 

 compared Alpine and Arctic plants Effects of cultivation 

 Local races of oysters and of snails Lepidoptera in Malay Archi- 

 pelago Local races of shrimps, of mackerel, and of herring 

 North American birds and mammals Action of climate on goats 

 and on rabbits Effect of domestication on rabbits, pigeons, fowls, 

 and ducks. 



IN the three preceding chapters we have examined 

 numerous cases of variation produced wholly or in great 

 part by a change in some one condition of environment. 

 In the present chapter no such attempt is made to trace 

 an effect to any single cause, but we shall examine the 

 effects of all conditions of life together, such as climate, 

 nutrition, moisture, and sunlight, in the production oi 

 variations. The variations more particularly to be 

 studied are those which are common to whole groups 

 of organisms, and which go to form what are known as 

 local races. Unfortunately in the majority of cases it 

 is impossible to prove that such races are the direct or 

 indirect results of the differences of environment, even 

 when there is a high probability that such is the 

 case. Hence, when local races are referred to, it is not 

 intended to imply that the distinguishing characters 

 exhibited are definitely due to the action of the environ- 



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