ZOOIC EVIDENCE. 319 



since the Devonian period. All well-preserved fossil trunks should afford 

 growth evidence of climatic cycles, except where annual rings were obscure or 

 lacking because of seasonal uniformity. It should not be impossible to obtain 

 some evidence of this sort for the Paleozoic and Mesozoic eras, while it should 

 be relatively simple for periods like the Miocene with its many petrified trees, 

 and the Pleistocene and Human with many tnmks buried in peat-bogs. 



ZOOIC EVIDENCE. 



Animal fossils. — The abundance and variety of animal remains in the geo- 

 logical record have made them of the first importance in fixing the sequence 

 of horizons. It may perhaps be doubted whether the marine forms are as 

 valuable in this connection as has been assumed. At any rate, it is clear that 

 they can furnish only indirect evidence of land conditions and of the nature 

 of terrestrial life. As the last member of the sequence, the correlations of 

 animal life with deformation, climate, and vegetation are pecuharly complex. 

 The land fauna is immediately dependent upon vegetation and climate, but 

 more remotely upon deformation. Marine life responds quickly to deforma^ 

 tion and circulation changes, but can be affected only indirectly by climate and 

 the vegetation of the land. The life of shallow epicontinental seas clearly 

 stands in much closer relation to climate, erosion, and vegetation, while that 

 of fresh-water bodies is largely or wholly controlled by them. As a conse- 

 quence, there arises the greatest variety of response, and hence of evidence. 

 The very complexity of the latter testifies to its value, since every sequence 

 has three or four effects, alike in kind but differing in degree and in proximity. 

 Whatever may be true of the present interpretations of marine fossils, it seems 

 clear to the ecologist that the correlation of the land vegetation and fauna has 

 scarcely begun, and that it must hold a future of peculiar attractiveness. 

 Natiu-aUy, a few of the outstanding relations, such as that of mammals to the 

 appearance of angiospenns and especially grasses, have been pointed out. 

 No adequate treatment of this subject is possible, however, until the interaction 

 of plant and animal communities at the present time is much better under- 

 stood. Indeed, it seems certain that this will involve not only the articulation 

 of distinct but associated plant and animal conununities, but the recognition 

 of actual biotic communities, in which certain plants and animals are at least 

 as closely and definitely interdependent as the plants or animals are among 

 themselves. It seems certain also that these biotic communities will prove 

 to have an organic development and structure, such as has already been 

 shown for plant formations. 



Culture relicts. — Paleo-ecology develops its most fascinating aspect when it 

 reaches the Human period. The sequence of cause and effect here attains its 

 greatest complexity and interest. Primitive man must have been peculiarly 

 dependent upon climate, vegetation, and animal life, though the effect of 

 vegetation must have increased as he passed from the nomadic to the agri- 

 cultiu-al stage. His local control of climate, i. e., by means of shelter, clothing, 

 fire, etc., must have been very much less than at present, but his powers of 

 adaptation must have been much higher. Human evidence of past climates 

 and biotic communities, or Monies, must come to be of very great value when 

 our methods of interpretation have improved. These wiU doubtless come to 

 deal more and more with community ciilture, and the conditions of climate and 



