234 POPULAR SCIENCE MONTHLY. 



sides are reaped from four to seven months after. But in the Delta, 

 rice, cotton, and indigo are sown in the spring (March or April) and 

 harvested in October, [November, and December. Here, irrigation 

 and temperature come in as disturbing elements, for the Delta feels 

 something of the cold of winter. 



I could give many other instances, but these will suffice. As a 

 general rule, we may say that in the temperate and frigid zones the 

 seasons for plants and animals are ruled by heat and cold, but that 

 in tropical and even in subtropical climates, rainfall and drought, 

 themselves largely due to the same circumstances, are the ruling 

 factors. 



Again, everybody knows that winter and summer, and the other 

 phenomena which simulate or accompany them, such as wet and 

 dry seasons, depend upon the fact that the earth's axis is not perpen- 

 dicular to the plane in which the earth moves round the sun, but 

 slightly inclined to it. ISTow, a year in itself, viewed as a measure of 

 time, is merely the period which it takes the earth to perform one 

 such complete revolution. During one half of each such revolution 

 the north pole is turned at a considerable angle toward the sun, and 

 during the other half, the south pole. "When the north pole is so 

 turned we call it summer in the northern hemisphere; when the 

 south pole is being favored, and the north is receiving less light and 

 heat, we call it winter. Let us suppose for a moment that the earth 

 had not got this twist or kink in its axis; that the equator was always 

 presented exactly toward the sun; what then would happen? Ob- 

 viously, there would be no change of seasons. The day and night 

 would have fixed lengths which never varied ; climate would in each 

 place be uniform and, barring accidents of elevation or distribution 

 of land and water, the climate of each place would also depend en- 

 tirely the whole year round on its distance from the equator. Rough- 

 ly speaking, the temperature of a district would be the temperature it 

 now possesses in March and September, only not quite so cold as 

 March nor so warm as September, owing to the absence of accu- 

 mulated heat from summer or of reserves of ice and snow from 

 winter. In one word, under such conditions there would have been 

 climates — marked belts of climate; but there would not have been 

 seasons. 



Seasons, however, depend in great part, as Mr. Alfred Russel 

 Wallace has ingeniously shown, on a great many things besides this 

 mere inclination of one end or the other of the earth toward the 

 sun in June and January. Much must be laid to the count of 

 accumulated stores of heat or cold; and though accumulated cold 

 is physically a misnomer, still for all practical purposes we may 

 apply the words fairly enough to the ice caps of the pole and the 



