THEORY OF EVOLUTION 489 



unit now recognized. If this speculation is followed further, it may 

 be supposed that the unicellular organisms of modern times have 

 descended from such early cells without changing from the simple- 

 celled state, but have developed many specializations as individual 

 types, while metazoans have advanced from certain of these simple 

 forms to a more complexly organized cellular condition. 



From the nature of protoplasm and from the evidence in the geo- 

 logical records, it is usually assumed that organisms first appeared 

 in the water near the shores of the primitive oceans. Presumably 

 the first oceans were boiling hot, and the land at that time was a 

 molten volcanic mass. If this is true, there must have been a long 

 period after the formation of the atmosphere, seas, and land before 

 protoplasm, as we know it, could have existed. After these cooled 

 and became favorable for life the seas are thought to have supported 

 an abundant life before the land became suitable for its existence. 

 Along with these several speculative aspects of the subject there 

 have been offered several forms of evidence to support the existence 

 of an evolutionary progress of development in organisms. 



Geological Evidence. — Paleontology is the study of fossil remains 

 of organisms deposited in the strata of the earth's crust. Shells 

 and other hard parts are mineralized or petrified; in other cases 

 mud impressions, or tracks, or pitch-preserved individuals, such as 

 insects, and a few frozen forms constitute the majority of fossils. 

 It must be kept clearly in mind that the geologist is able to deter- 

 mine quite accurately the sequence of time or chronological suc- 

 cession of the layers of the earth's crust. The geological time scale 

 shows a long period before any life existed, then the appearance of 

 unicellular plants, then unicellular animals, then colonial forms, 

 simple many-celled forms, and then the more complex ones. Such 

 a timetable estimates the relative period of time during each era 

 and shows some fusion and overlapping of certain types of life. 

 Certain types of unicellular forms are continuous through the en- 

 tire scale. 



The principal facts shown by the fossil record may be summa- 

 rized: (a) The fossil forms are not strictly identical with any living 

 species, and the remains of plants and animals of each geologic 

 stratum are at least specifically different from the forms in any 

 other stratum, but they may belong to the same genus; (b) the 

 oldest strata containing fossils have represented in them most of 



