6 ' INTRODUCTION 



offspring; although neither of these definitions is quite accurate. 

 From the tune when his attention was first directed to such 

 matters, man must have noted these facts of difference and resem- 

 blance and considered their significance as applied to his own 

 offspring or to his domesticated animals. In recent years, how- 

 ever, variation and heredity have come to be studied by them- 

 selves in what is known as the science of Genetics, which may be 

 said to have originated when the Mendehan laws of heredity were 

 rediscovered and became generally known, about the year 1900. 

 The relationship of Genetics to other biological sciences is very 

 extensive, as will be apparent upon reflection. Variations of form 

 are frequently studied, and hence an aspect of Morphology is 

 involved. There are also functional variations, which involve 

 Physiology. Embryology reveals the manner in which hereditary 

 features are passed from one generation to another. Taxonomy is 

 involved, because variations determine the limits of species in 

 classification. Moreover, Genetics is the key to the causes of 

 Organic Evolution, since evolutionary changes must originate in 

 variations and be perpetuated by heredity. As with the other 

 groupings of related sciences shown by the table, the list of those 

 bearing upon the science of Genetics might be greatly extended. 



Origin of the Race. — In ancient times the origin of the many 

 different kinds of animals was commonly ascribed to some form of 

 creation by which they were produced in their existing states, 

 unless indeed it was believed that each individual arose sponta- 

 neously. With the advance of knowledge concerning animal and 

 plant life, beUef in an evolutionary process became inevitable. 

 Organic Evolution is the term applied to the transformations of 

 living things since they appeared upon our planet. It may be com- 

 pared with Inorganic Evolution, which describes the history of 

 non-living things in theories concerning the transformations of 

 solar systems or of chemical elements, or in the more certain theories 

 of geologic evolution. 



Organic Evolution is the biologist's answer to the question of 

 the historical development of the many and varied forms of life. 

 Within the past century it has come to be supported by over- 

 whelming evidence as the most reasonable explanation of this 

 historic course of events. The sciences of Morphology, Physiology, 

 Embryology, Zoogeography, Paleontology, Ecology, and Taxonomy 

 are important to the student of Organic Evolution. Comparative 



