DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 1 39 



time, with geographic and climatic changes, the living things 

 were changing both in their nature and their position on the 

 earth. Thus during the various geologic periods, the distribu- 

 tion and character of the life of one period determines the life 

 of the next. 



All our knowledge of the life of the earlier times is gained 

 from fossil remains found in the limestone, sandstone, clay and 

 other strata of rock. Of course only the hard parts can be 

 preserved, and only a small proportion of these are found in a 

 form to give us much information. Notwithstanding, we are 

 able to get from the strata a very fair idea of the progress of 

 life on the globe. In the earliest fossil-bearing strata we find 

 only invertebrate remains. The invertebrates have continued 

 through all the successive strata to the present time, but in 

 doing so they increase in differentiation and become more and 

 more like present invertebrates. Of the vertebrates the fishes 

 appeared first, then the amphibians, reptiles, mammals, and 

 birds. None of these when they first appeared were like their 

 modern successors. As we pass upward through the strata, 

 old species become extinct and new ones, more and more like 

 the species of the present, arise from them, presumably by the 

 changes made necessary in becoming adapted to the changing 

 earth conditions. In a general way the fossils of any age are 

 intermediate — "connecting links" — between those of the ages 

 preceding and following. In other words plants and animals 

 have gradually been making progress toward present forms 

 through all these ages. The study of this department of the 

 adaptation of animals is known as PalcBozoology. 



173. Summary. 



1. It is necessary to consider the individual not merely as 

 a group of cells and tissues but as a unit acting and being acted 

 upon by all external forces and by other organisms. 



2. Characteristics derived from the germ cells of parents, 

 whether resulting in similar qualities or in new ones, are de- 

 scribed as hereditary. The reproductive cells are the carriers 

 of ancestral qualities. 



3. Individuals vary as the result (i) of internal conditions 

 and changes, the causes of which are obscure, and (2) of differ- 



