0. — GEOLOGY. Bb 



the nature of the food. Eoughly to summarise, he points out that 

 from the Eocene onwards the sirenians underwent a steady, slow change, 

 because, though they migrated from land to sea, they retained their 

 habit of feeding on the soft water-plants. The horses, though they 

 remained on land, display an evolution at fiist rather quick, then 

 slower, but down to Pliocene times always quicker than that of the 

 sirenians; and this is correlated with their change into eaters of grain, 

 and their adaptation to the plains which furnish such food. The whales, 

 like the sirenians, migrated at the beginning of the Tertiary from land to 

 sea; but how different is their rate of evolution, and into what diverse 

 forms have they diverged ! At first they remained near the coasts, keep- 

 ing to the ancestral diet, and, like the sirenians, changing but slowly. 

 But the whales were flesh-eaters, and soon they took to hunting fish, and 

 then to eating large and small cephalopods ; hence from the Oligocene 

 onwards the change was veiy quick, and in Miocene times the evolution 

 was almost tempestuous. Finally, many whales turned to the swallow- 

 ing of minute floating oi'ganisms, and from Lower Pliocene times, 

 liaving apparently exhausted the possibilities of ocean provender, they 

 changed with remarkable slowness. 



Whether such changes of food or of other habits of life are, in a 

 sense, spontaneous, or whether they are forced on the creatures by 

 changes of climate and other conditions, makes no difference to the 

 facts that the changes of form are a reaction to the stimuli of the outer 

 world, and that the rate of evolution depends on those outer changes. 



Whether we have to deal with similar changes of form taking place 

 at different times or in different places, or with diverse changes affect- 

 ing the same or similar stocks at the same time and place, we can see 

 the possibility that all are adaptations to a changing environment. 

 There is then reason for thinking that ignorance alone leads us to assume 

 some inexphcable force urging the races this way or that, to so-called 

 advance or to apparent degeneration, to life or to death. 



The Rhythm of Life. 



The comparison of the life of a lineage to that of an individual is, 

 up to a point, true and illuminating ; but when a lineage first starts on 

 its independent course "(which really means that some individuals of a 

 pre-existing stock enter a new field), tlien I see no reason to predict 

 that it will necessarily pass through periods of youth, maturity, and 

 old age, that it will increase to an acme of numbers, of variety, or of 

 specialisation, and then decHne through a second childhood to ultimate 

 extinction. Still less can we say that, as the individuals of a species 

 have their allotted span of time, long or short, so the species or the 

 hneage has its predestined term. The exceptions to those assertions 

 are indeed recognised by the supporters of such views, and they are 

 explained in terms of rejuvenescence, rhythmic cycles, or a "-rand 

 despairing outburst before death. This phraseology is dehghtful as 

 metaphor, and the conceptions have had their value in promoting search 

 for confirmatory or contradictory evidence. But do they lead to any 

 broad and fructifying principle? When one analyses them one is per- 

 petually brought up against some transcendental assumption, some 



