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 phytogeny. 
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 
