488 ESSENTIALS OP ZOOLOGY 



Basis for the Theory of Evolution 



One who lias thoughtfully studied the field of zoology soon realizes 

 many relationships or homologies in structural make-up of certain 

 different groups of animals ; much similarity of embryonic develop- 

 2iental processes, and fundamental coincidences in physiological 

 activities in all living material. Too, it is recognized that fauna 

 and flora are not the same in all parts of the earth at the present, 

 and have not been the same in the past as now. These realizations 

 and other similar ones have been based on scattered sources of evi- 

 dence, much of which is indirect. The evidences which have been 

 discovered, however, have led to the formulation of the theory of 

 evolution and its rather general acceptance as a working hypothesis 

 among biologists. There has been confusion regarding this subject 

 through failure to distinguish between the existence of progressive 

 evolutionary changes, the course of these changes, and the cause of 

 such a progressive series of events. The latter point has been the 

 basis of most of the questions concerning the whole idea, and it is 

 the most speculative of the three. These three are separate though 

 related factors. 



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, 

 coincident 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 



