DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 137 



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 distribution 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 are like 

 their modern types. 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 chang- 

 ing 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 been making progress toward present forms 

 through all these ages. The study of this department of 

 adaptation of animals is known as Pal&ozoology. 



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- 



