504 STRUCTURE AND LIFE HISTORIES 



has come to be, by a series of gradual changes. Rivers 

 and valleys are constantly changing so that the present 

 landscape is the result of evolutionary processes; climates 

 have changed, as we know from the fact that fossil re- 

 mains of tropical plants are now found in the rocks in 

 arctic regions; even the stars and planets, like our own 

 earth, are coming gradually into being, undergoing changes 

 of surface and interior condition, and ceasing to exist. 

 Nothing is constant except constant change. The main 

 problem of astronomy is to ascertain and record, in order, 

 the evolutionary changes that have resulted in the present 

 system of suns and planets. The main problem of 

 geology is to ascertain and record, in order, the evo- 

 lutionary steps that have resulted in the present condition 

 of the earth. 



437. Organic Evolution. Developmental changes in 

 living things constitute organic evolution. Such changes 

 are manifested in the development of an individual from 

 a spore or an egg. The development of a mature in- 

 dividual is ontogeny. The development of a group of 

 related forms (genera, families, orders, etc.) is phylogeny. 

 The chief problem of biology is to ascertain and record, 

 in order, the evolutionary changes that have resulted in 

 the appearance of life and the present condition of living 

 things. 



The major problem of botany is to record, in order, the 

 evolutionary steps that have culminated in the present con- 

 dition of the plant world. 



Organic evolution means that, after the first appearance 

 of life, all living things, plant or animal, have been 

 derived from preexisting living things, in other words, that 

 the present method of formation of living things, by the 



