14 EVOLUTION OF LIFE 1.9 



importance to the biologist. We can hardly expect to treat animals 

 and plants as stable systems if the environment around them is 

 changing. In order to be able to assess the influence of such changes 

 on life we must know more about the rates at which they occur, and 

 careful study shows that some of the climatic changes are rhythmic. 

 Rhythmic changes of climate are, of course, very familiar to us in the 

 cycles of days, months, and years, and the immense importance of 

 these short-period changes for animal and plant life must not be 

 forgotten. 



Here we are more concerned with changes of longer periodicity, of 

 which the best known are fluctuations of the amount of solar radiation 

 received at any given part of the earth's surface. These are likely to 

 be especially important since plants, and hence ultimately animals, 

 depend for their energy on sunlight. The cycle of number of sun- 

 spots (n*4 years) involves a change in amount of radiation, and this is 

 associated with some biological cycles, for instance in the distribution 

 of the rings of growth made by trees. Longer-period fluctuations in 

 the amount of radiation received on any part of the earth's surface 

 depend on the perturbations of the earth's orbit, particularly on 

 changes in the obliquity of the ecliptic. The effect of these perturba- 

 tions can be calculated, and the results show that at any one place 

 there are rhythmical variations in the amount of radiation received, 

 and in its seasonal distribution. The periodicity of these calculated 

 changes is about 40,000 years, with considerable irregularities and 

 variations in the sizes of the maxima (Fig. 2). 



During the last million years (the Pleistocene epoch) there has been 

 a series of waves of glaciation (ice ages); the ice-caps have several 

 times advanced towards the equator and then retreated again. These 

 changes are usually classified into four periods of glaciation, separated 

 by interglacial periods. However, the last (fourth Pleistocene) glacia- 

 tion, of which we know the most, certainly had three separate climaxes 

 of cold. The correspondence of these with especially marked minima 

 in the curve of solar radiation is not perfect (Fig. 2), but it suggests 

 that the basic periodicity may have been something like 40,000 years, 

 and that the division of the whole Pleistocene period into four periods 

 of glaciation obscures a change with much shorter periodicity. From 

 about 120,000 to 180,000 years b.p. (Before Present) there were no 

 marked minima in the solar radiation curve, and this agrees with other 

 evidence of a long interglacial period (third Pleistocene interglacial). 

 Two marked minima agree with the other signs of a penultimate (third 

 Pleistocene) glaciation, and this was preceded by a very long warmer 



