FIG. 10-5 Response of the hypotrlchian Oxyincba fallax to heat. 

 The slide is heated at X. An Oxytricha at station I changes 

 position as indicated by the arrows, repeatedly moving back- 

 wards, turning to the right, then moving forwards. Finally, at 

 stations 13-14, it directs itself away from the heat, and moves 

 in a straight path towards a cooler region (from Jennings 1906). 



Trial and error is involved to a considerable ex- 

 tent in the dispersal movements of animals. If, ini- 

 tially, an individual progresses into an unfavorable 

 habitat, it may be able to withdraw and proceed in a 

 new direction. This may continue many times until 

 by chance it discovers an area that is favorable. Trial 

 and error movements are manifested in all groups of 

 animals. In higher types, and even in many lower 

 types, the individual may learn by experience and re- 

 duce the number of false trials that it makes. Thus 

 initially random movements may eventually evolve 

 into directed behavior patterns. 



Barriers 



Dispersal continues until a barrier that makes 

 further movement difficult or prevents successful col- 

 onization is encountered. Barriers are of different 

 sorts and are classified as physiographic . climatic, and 

 biotic. The difficulty of surmounting or bypassing 

 any type of barrier varies greatly among species. 

 River valleys, for instance, may be barriers to animals 

 frequenting mountains ; but to lowland species, river 

 valleys are important dispersal highways. 



To fresh- water organisms in river and lake habi- 

 tats, intervening land masses are usually effective bar- 

 riers. The headwaters of different river systems may 

 lie only a few miles apart, yet contain quite different 

 species because the continuous water route down tu 

 the mouth of one river and back up to the other may 

 be a distance of several hundreds or thousands of 

 miles. A waterfall may be a barrier for all non-flying 

 aquatic species, and even riffles or swift water may 

 prevent upstream dispersal of pond or lake species. 

 The salt water intervening adjacent rivers flowing 

 into the sea is a barrier to most fresh-water forms 

 that might otherwise invade one river from the other. 



Terrestrial organisms are hemmed in by a great 

 variety of barriers. The oceans constitute the major 

 physiographic barriers since they separate the faunas 

 of the several continents and isolate islands from each 

 other. Lakes are not effective barriers because they 

 can be readily skirted, but wide rivers coursing long 

 distances between banks of dense vegetation, as does 

 the Amazon River through the tropical rain forests 

 of South America, may limit the range of forest mam- 

 mals, butterflies, flightless beetles, land snails, and 

 even birds (Mayr 1942: 228-9). The Grand Canyon 

 of the Colorado River separates the ranges of the 

 Kaibab and Abert squirrels and several other species, 

 even in country that is semi-arid and open (Goldman 

 1937). Mountains are sometimes considered barriers 

 to lowland species, and valleys barriers to mountain 

 forms. This is only true if the change in climate and 

 vegetation that such barriers produce are unfavorable 

 to the species. 



Deserts are important climatic barriers since they 

 are hot, and dry. Temperature affects animals di- 

 rectly, since species have definite limits of tolerance, 

 comfort, and efficiency. Precipitation is important be- 

 cause it controls the type of vegetation that occurs in 

 a region, and by so much the animals adapted to that 

 type of vegetation. A low relative humidity may di- 

 rectly limit the dispersal of moist-skinned species. 

 Excessive solar radiation limits some species to forest 

 habitats, excluding them from open country ; without 

 recourse to shade, such animals would experience 

 overheating and critical loss of water from the body. 

 Short photoperiods may limit distribution northward 



148 Ecological processes and dynamics 



