

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 871 



In general, scientists are convinced, according to the evidence 

 they have examined, that organic evolution exists in the form of a 

 progressive change, which has generally proceeded from simpler to 

 more complex organization through a long period of time. Although 

 some sections of the course of this series of developmental events 

 cannot be charted as completely as a ship's course at sea, numerous 

 landmarks serve to indicate what has taken place. Biologists are 

 free to admit that the causes of this process are not understood, and 

 treat it strictly as a scientific problem. 



Most estimates of the time when living organisms first came into 

 existence range between 60,000,000 and 1,200,000,000 years ago, co- 

 incident with Pre-Cambrian deposits. The manner of origin of 

 protoplasm is purely hypothetical. It is suggested that when con- 

 ditions became suitable, as to chemical elements present, tempera- 

 ture, pressure, etc., a relatively simple colloidal, protoplasmic mix- 

 ture arose, having properties of life though perhaps in a very ele- 

 mentary way. It is assumed that all organisms which have lived 

 are descendants of this simple origin. Cellular organization, with 

 division of labor between nucleus and cytoplasm, is taken to be a 

 first step in the development of protoplasm. It is the simplest vital 

 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 orgajiisms 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 sup- 

 ported an abundant life before the land became suitable for its 

 existence. Along with these several speculative aspects of the sub- 

 ject there have been offered several forms of evidence to support the 

 existence of an evolutionary progress of development in organisms. 



