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been described above (Introduction). Finally, a new field of 

 research is growing around the species. 



The Science of the Species 



It has become apparent within recent years that a new branch 

 of biology is developing, the science of species. It is devoted to 

 the many-sided aspects of the species and to the understanding 

 of the level of integration which is denoted by the term species. 

 This branch of biology is as legitimate as is cytology, devoted to 

 the level of the cell, or histology, dealing with the level of the 

 tissue. The species is an important unit in evolution, in ecology, 

 in the behavioral sciences, and in applied biology. 



The study of the biological properties of species has within 

 recent decades led to the development of several new fields in 

 biology, each being a borderline field between the science of 

 species on the one hand and some other branch of biology, e.g., 

 genetics, ecology, animal psychology on the other. It would lead 

 too far to paint a detailed picture of this recent development, and 

 I will content myself to outline it with a few bold strokes. These 

 new fields are: 



The Study of Speciation. Darwin, as we saw, had fallen down 

 in his attempt to explain the multiplication of species because he 

 was not fully aware of the multiple aspects of species. Nor did 

 he realize that multiplication of species, an origin of discontinui- 

 ties, is not the same as simple evolutionary change. The study 

 of the mechanisms and modes of speciation has become an inter- 

 esting borderline field between systematics, genetics, and evolu- 

 tion. 



The Study of Isolating Mechanisms. The appreciation of the 

 fact that in addition to sterility there are many other factors 

 which safeguard the genetic integrity of species has led to the 

 development of a new field of study. As far as animals are con- 

 cerned, contact was established with the field of animal psychol- 

 ogy because the ethologieal isolating mechanisms are among 

 the most conspicuous manifestations of animal behavior. The 

 study of courtship behavior has importantly contributed to the 



