FIFTY YEARS' PROGRESS IN SCIENCE. 465 



At this stage Man appears to have been ignorant of pottery, to have had no 

 knowledge of agriculture, no domestic animals, except perhaps the dog. His 

 weapons were the axe, the spear, and the javelin ; I do not believe he knew the 

 use of the bow, though he was probably acquainted with the lance. He was, of 

 course, ignorant of metal, and his stone implements, though skilfully formed, 

 were of quite different shapes from those of the second Stone- age, and were never 

 ground. This earlier Stone period, when man coexisted with these extinct mam- 

 malia, is known as Palaeolithic, or Early Stone Age, in opposition to the Neolith- 

 ic, or Newer Stone Age. 



The remains of the mammalia which coexisted with man in prehistoric times 

 have been most carefully studied by Owen, Lartet, Riitimeyer, Falconer, Busk, 

 Boyd-Dawkins, and others. The presence of the mammoth, the reindeer, and 

 especially of the musk-ox, indicates a severe, not to say an arctic, climate, the 

 existence of which, moreover, was proved by other considerations : while, on the 

 contrary, the hippopotamus. requires considerable warmth. How then is this as- 

 sociation to be explained ? 



While the climate of the globe is, no doubt, much affected by geographical 

 conditions, the cold of the glacial period was, I believe, mainly due to the ec- 

 centricity of the earth's orbit combined with the obliquity of the ecliptic. The 

 result of the latter condition is a period of 21,000 years, during one-half of which 

 the northern hemisphere is warmer than the southern, while during the other 

 10,500 years the reverse is the case. At present we are in the former phase, 

 and there is, we know, a vast accumulation of ice at the south pole. But when 

 the earth's orbit is nearly circular, as it is at present, the difference between the 

 two hemispheres is not very great ; on the contrary, as the eccentricity of the 

 orbit increases the contrast between them increases also. This eccentricity is 

 continually oscillating between certain limits, which Croll and subsequently Stone 

 have calculated out for the last million years. At present the eccentricity is -016 

 and the mean temperature of the coldest month in London is about 40°. Such 

 has been the state of things for nearly 100,000 years; but before that there was 

 a period, beginning 300,000 years ago, when the eccentricity of the orbit varied 

 from •26 to 57. The result of this would be greatly to increase the effect due to 

 the obliquity of the orbit; at certain periods the climate would be much warmer 

 than at present, while at others the number of days in winter would be twenty 

 more, and of summer twenty less than now, while the mean temperature of the 

 coldest month would be lowered 20°. We thus get something like a date for the 

 last glacial epoch, and we see that it was not simply a period of cold, but rather 

 one of extremes, each beat of the pendulum of temperature lasting for no less 

 than 21,000 years. This explains the fact that, as Morlot showed in 1854, the 

 glacial deposits of Switzerland, and, as we now know, those of Scotland, are not 

 a single uniform layer, but a succession of strata, indicating very different condi- 

 tions. I agree also with Croll and Geikie in thinking that these considerations 

 explain the apparent anomaly of the coexistence in the same gravels of arctic and 



