386 DIFFICULTIES AND IMPORTANCE OF THE CONCEPT 



concepts of Lorenz and Tinbergen and has led to a reinterpreta- 

 tion of many facts which Darwin had grouped together under 

 the heading "Sexual Selection." 



The Study of Intraspeciftc and Interspecific Competition. The 

 realization that there is a subtle difference between the compe- 

 tition that goes on among conspecific individuals and that among 

 individuals belonging to different species has had a very stimulat- 

 ing effect in the field of ecology. That interspecific competition 

 is an important centrifugal factor in evolution has been dem- 

 onstrated by various recent authors. Attempts to determine and 

 to measure such competition more precisely have led to a much 

 more detailed study of the ecology and the population structure 

 of species than was attempted by the ecologists of preceding 

 decades. 



Study of the Genetic Structure of Species. The realization 

 that local populations belong to a broader system, the species, 

 and that the total genetic content of these intercommunicating 

 populations are the gene pool of the species has had a profound 

 effect on population genetics. It has led to new questions and 

 has facilitated the understanding of certain intrapopulation 

 phenomena. Studies of the balance of the gene complex, of the 

 balance between local adaptation and compatibility with gene 

 dispersal and of hybridization between species have led to a 

 broadening of genetics which transcends considerably the ge- 

 netics of the early Mendelian period. 



The Study of Physiological Species Differences. In physiology 

 a development is taking place which parallels that in genetics. It 

 involves the realization that the ecotypic physiological differ- 

 ences of local populations are of a different order of magnitude 

 from that existing between good species. Each species is a sep- 

 arate physiological system, and these physiological differences are 

 of different kinds as pointed out by Prosser (in this symposium). 

 With physiological characteristics of populations, special care 

 must be exercised to distinguish between genetically controlled 

 characters and others. Prosser lias shown that nongenetic ac- 

 climatizations are particularly frequent in marine animals with 



