12 ANCESTRAL MAN. 



factor in the production of an ice age, but there are others of 

 more importance. "The climate," says Prof. Dawkins, "must 

 " have been arctic in its severity during this period of glaciation, 

 " and this may have been partially due to the fact of the land 

 " standing at a higher level above the sea, and being lifted up 

 " into the colder regions of the atmosphere. It cannot however 

 " be wholly so explained, since it was the culmination of a series of 

 " changes by which the tropical climate of the Eocene passed 

 " into the warm Meiocene and the temperate Pleiocene climates." 



The one source of heat is the sun, the chief occasion of cold is 

 radiation ; what we have to discover, then, is some reciprocating 

 causal action that shall both produce the heat and permit the cold. 



It is well known that the earth's orbit undergoes gradual 

 changes, becoming at one time almost circular, and at another 

 considerably eccentric. So much is- this the case, that the 

 difference between her nearest distance to the sun, and hex farthest 

 distance from the sun may be upwards of fourteen millions of 

 miles (14,368,200). 



This change in the eccentricity of the earth's orbit may affect 

 the climate in two ways ; 1. — By increasing or diminishing the 

 mean annual amount of sun-heat ; or, 2. — By increasing 

 or diminishing the difference between summer and winter 

 temperatures. 



1. — The total quantity of sun-heat received by the earth in a 

 year is inversely proportional to the minor axis of her orbit, so 

 that the lessened amount of heat occasioned by her maximum 

 eccentricity would be only as 997 to 1,000 ; or, in other words, 

 the sun-heat which is now sufficient to melt 100 feet of ice would 

 then melt 3^ inches less. It must not be forgotten, however, 

 that the sum of constantly recurring, though minute, increments, 

 may reach at length overwhelming proportions. 



Prof. Haughton shows that at a North Latitude of 70° the 

 greatest possible lowering of the mean secular temperature 

 occasioned by the maximum eccentricity would be 4.985 F. 

 But he is of opinion that a diminution of 4° F. in the temperature 

 of the summer months of Norway, would place one-fourth of the 

 -surface of that country within the snow-line, and would pour 

 glaciers into the head of every fiord that opens on the western 

 coast. As the figures he gives are those of mean temperatures, it 

 is easy to understand that by diminished annual sun-heat alone, as 

 occasioned by terrestrial eccentricity, there might come upon 

 countries north of 50 lat. a very considerable refrigeration. 



2. — A change in the eccentricity of the earth's orbit may 

 influence the climate by increasing the difference between summer 

 and winter temperahires. A period of maximum terrestrial eccen- 

 tricity may last for 100,000 or 200,000 years, and meanwhile, in 



