NATURAL SELECTION 



659 



existence vary with time: one species may 

 be favorable for the growth of another at 

 the beginning of the experiment, and the 

 depression of one species by another will 

 only begin later. 



Under natural conditions, if the food 

 production of a given habitat remains con- 

 stant, the biomass tends to remain constant, 

 with increase of one species correlated with 



foods (Lack, 1944, 1945a, 1946, 1947; 

 Amadon, 1947; Mayr, 1948; see p. 369 

 for finrther discussion). Examples of 

 birds that breed in a similar habitat, but 

 occupy different regions, are the swans, 

 Cygnus olor and C. cygnus, the curlews, 

 Numenius arquata and N. phaeopus, and 

 the Common and Arctic terns. Sterna hirun- 

 do and S. macrura. In each case the first- 



Fig. 240. Divided range of Butler's garter snake (Thamnophis butleri) (eastern shaded 

 area) produced by the invading Plains garter snake (Thamnophis radix) (western shaded 

 area). (After Pope.) 



a decrease in competing species. Hubbs 

 and Eschmeyer (1938) indicate that the 

 weight of living fishes in a lake remains con- 

 stant, although fluctuations may occur in 

 the weight of each competing species. 

 When a species is introduced into a new 

 region, the growth of individuals is some- 

 times exceptionally great because the 

 small number of individuals results in a 

 minimum of competition within the species 

 (Eddy and Carlander, 1940). 



The majority of closely related species 

 with similar ecological adjustment inhabit 

 different areas or habitats, or eat different 



mentioned species breeds to the south of 

 the second. Lack reports examples of re- 

 lated species that breed in the same regions, 

 but in different habitats. These include the 

 long-eared owl, Asio otiis, in woods, and the 

 short-eared owl, A. fammetis, in open 

 country; the marsh harrier. Circus aerugin- 

 osus, in wet marshland, and the Montagu 

 harrier, C. pygofgus, in drier marshland 

 and heaths; the goosander, Mergus mer- 

 ganser, mainly near rapid streams, and the 

 red-breasted merganser, M. serrator, mainly 

 near lakes or bays; the common European 

 hare, Lepus europaeus, on low ground, and 



