Segregation 53 



ertson (1906) says, "Species are characterized by non-competitive 

 habits rather than adaptive structures." Certain plethodontid sala- 

 manders segregate most sharply into habitats during their breeding 

 periods. They avoid competition by selecting different humidities, 

 different types of streams, etc. (Noble, 1927). Vestal (1914) has 

 gone so far as to enumerate five types of characters which remove 

 animals from competition: (1) structural, which give animals spe- 

 cial advantages in particular habitats (as the legs of a mole) ; (2) 

 physiological, such as ability to digest and assimilate unusual foods 

 (clothes moth eats keratin) or to exist in peculiar environments 

 (anaerobic animals) ; (3) psychological, such as preferences for 

 special foods or habitats; (4) biological, which permit adaptation 

 of life cycles to favorable seasons, feeding to particular times of 

 day, etc.; and (5) numerical by which the numbers of a species are 

 adjusted to food supply or other limiting environmental factors. 

 When a new area is opened for population, competition may be sus- 

 pended for a time (Borradaile, 1923). Overpopulation may lead 

 to extreme competition, epidemics, and consequent decrease in num- 

 bers. Thus a small group of peculiar individuals may survive the 

 period of stress and initiate a new race or species (Elton 1924) . 



Another way in which animals may segregate is through assortive 

 mating (Poulton, 1908). Fulton (1933) recently separated three 

 races of the cricket Nemobuis fasciatus (De Geer) chiefly because 

 they have different songs. These races "are more distinct physio- 

 logically than morphologically," and "they seldom if ever interbreed 

 under natural conditions." Probably there are other animals which 

 have formed special groups and taken up life in particular habitats 

 because of peculiar secondary sex characters. "The evolution of 

 secondary sex characters is usually not progressive and continuous 

 but hap-hazard and often parallel in not closely related stocks" 

 (Noble, 1927) . Perhaps it has permitted groups of animals to 

 become adapted to new habitats. 



