E. MAYR 13 



are merged into a single one. It was pointed out above that 

 males and females are often exceedingly different "kinds" of ani- 

 mals. Even more different are in many animals the larval stages, 

 or in plants sporophyte and gametophyte, or in polymorph popu- 

 lations the various genotypes. A strictly morphological-typological 

 concept is inadequate to cope with such intraspecific variation. 

 It is equally incapable of coping with another difficulty, namely 

 an absence of visible morphological differences between natural 

 populations which are nevertheless distinct and reproductively 

 isolated, and therefore to be considered species. The frequent 

 occurrence of such "cryptic species" or "sibling species" in nature 

 has been substantiated by various genetic, physiological, or 

 ecological methods. They form another decisive argument against 

 defining species on a primarily morphological basis. Any attempt 

 in these two situations to define species "by degree of difference" 

 is doomed to failure. Degree of difference can be specified only 

 by a purely arbitrary decision. 



More profound than these two essentially practical considera- 

 tions is the fact that the typological species concept treats species 

 merely as random aggregates of individuals which have the "es- 

 sential properties" of the "type" of the species and "agree with 

 the diagnosis." This static concept ignores the fact that species 

 are not merely classes of objects but are composed of natural 

 populations which are integrated by an internal organization and 

 that this organization (based on genetic, ethological, and eco- 

 logical properties) gives the populations a structure which goes 

 far beyond that of mere aggregates of individuals. Even a house 

 is more than a mere aggregate of bricks or a forest an aggregate 

 of trees. In a species an even greater supraindividualistic cohesion 

 and organization is produced by a number of factors. Species are 

 a reproductive community. The individuals of a species of higher 

 animals recognize each other as potential mates and seek each 

 other for the purpose of reproduction. A multitude of devices 

 insures intraspecific reproduction in all organisms. The species 

 is an ecological unit which, regardless of the individuals of which 

 it is composed, interacts as a unit with other species in the same 

 environment. The species, finally, is a genetic unit consisting of a 



