10 HISTORICAL GEOLOGY 



because a given species now lives under warm climatic conditions, 

 every species of the same genus has lived under similar conditions. 

 When, however, we are dealing with species still living, or in older 

 rocks, with whole groups of organisms pointing to certain climatic 

 conditions, we are reasonably safe in our inferences. 



4. Relations and distribution of modern organisms. It is evi- 

 dent that, if we are to properly understand the present-day rela- 

 tions and distribution of organisms, we must learn about their 

 ancestry and history, because all modern plants and animals have 

 directly descended from those which lived in earlier geologic epochs. 

 In many cases existing plants or animals, notably different in 

 structure, can be traced back to a common ancestry. Again, cer- 

 tain peculiarities in the distribution of some of the present-day ani- 

 mals are readily explained in the light of their geologic ancestry 

 and habitats. A good example is Australia, where practically all 

 of the present-day Mammals (barring those introduced by Man) 

 are of very simple types, that is, non-Placentals such as the Kanga- 

 roo, Spiny Ant-eater, etc., found only in and close to Australia, and 

 which are clearly much more like the Mammals of distinctly earlier 

 geologic time than like typical Mammals of the present day. The 

 explanation is that Australia was separated from Eurasia before 

 the higher (Placental) Mammals had been evolved, and that the 

 very different, or probably much less severe, struggle for existence 

 in isolated Australia has not been favorable for the evolution of 

 Placentals as was the case elsewhere. 



Outline Classifications of Animals and Plants 



Since a knowledge of the classifications of animals and plants 

 and the principal characteristics of the more important groups of 

 organisms is a fundamental consideration in the study of the life 

 of each period, and in understanding the bearings of these life 

 records upon the great doctrine of organic evolution, outline classi- 

 fications of plants and animals, with simple explanations, are here 

 given. The classifications are necessarily very brief, and no great 

 degree of biologic refinement is intended. Rather the purpose is 

 to have a convenient arrangement, essentially in biologic order, of 

 the principal groups of organisms to form a simple basis for the 

 discussion of the life of each period of geologic history as presented 

 in this text-book. 



