296 THE SURVIVAL OF THE UNLIKE. [XVII. 



although the miuor distributions withiu these zones 

 are dependent upon a multitude of climatal and 

 physical causes. The method of determining or 

 expressing the physiological unit with which these 

 zonal bounds are associated, has been most satisfac- 

 torily presented recently in an admirable paper by 

 Merriam,* who concludes that "animals and plants 

 are restricted in northward distribution by the total 

 quantity of heat during the season of growth and 

 reproduction," and that in their southward distri- 

 bution they are restricted by ' * the mean temperature 

 of a brief period covering the hottest part of the 

 year." If it is true that the northward spread of 

 plants is determined by the sum -total of heat units 

 which the plant receives, then it must follow that 

 when a plant is grown northwards beyond its own 

 zone, in a region of lower sum -temperature, it must 

 in some manner conform itself to that lower tem- 

 perature. If it adapts itself completely to the lower 

 sum -temperature, — as we find it usually does if the 

 change is not too violent, — then it becomes more 

 sensitive to small increments of temperature than it 

 was originally. It should vegetate and bloom rela- 

 tively earlier in spring at the north than at the 

 south ; and I shall now show, by experiment, that 

 this is usually the case. 



Cuttings and seeds taken from the south to the 

 north vegetate and germinate later than those 

 grown at the northern station. One of the best 

 experiments in this direction was made by Alphonse 



•C. Hart Merriam, " Laws of Temperature Control," etc., Nat. Geogr. Mag. vl. 

 229-238. aud charts. 1894. 



