AMERICAN INSTITUTE. 487 



soutliern period of his year, when he feeds on rice, he is the tit 

 bit of epicures. Climate produces little difference in this case, 

 as the bird migrates to avoid extremes of temperature. 



Some additional light may be thrown on this matter by con- 

 sidering the relation between the food of the animal and its con- 

 dition as expressed in the forces resulting from that condition, 

 Tliere are some millions of miles of travel annually run by stage 

 coaches in this country. The labor is performed during every 

 day in the year, and in extremes of temperature from 30'^ below 

 zero to 100'^ above that point — a variation equal to the average 

 difference between polar and equatorial climates. Now this wide 

 experiment settles some facts of much interest, such as that the 

 amount of grain required to do a mile of travel is very nearly 

 definite, being aboiit a quart of oats or half as much maize per 

 animal. During winter and summer the food does not vary, but 

 during summer the animals throw off the surplus carbonaceous 

 portion in the form of sweat. There is no such thing known in 

 this country as a stage line maintained on hay and root feed, and 

 that fact is significant of a deep and broad physiological law. 

 Horses at pasture are well known to be unfit for any long con- 

 tinued severe labor, their flesh being soft and becoming harder 

 only after the use of grain. Grain fed animals also have a higher 

 market value in the shambles, corn fed pork than milk fed pork, 

 and corn fed beeves than grass fed beeves. How insignificant is 

 the influence of climate w^hen compared with food in all these 

 cases .! 



CONDITIONS OF VARIATION IN PLANTS. 



As we have found the cause of the variation in animals to lie 

 in the food which is appropriated in different ways, and to differ- 

 ent extents, by climate and exercise, so we shall find the great 

 cause of variation in the plant to be the same, and that climate 

 merety indicates tJie amount of exercise of the plant. It is true that 

 the general distribution of plants on the earth's surface is in 

 broad belts, corresponding with the parallels of latitude; the 

 centre of the belts being most developed, and the sides less, until 

 they gradually merge into other floral zones. This general dis- 

 tribution depends on the law of Bousingault, that the sun's forces 



