-Ml animals are subject to predatioii or coinpeti- 

 tioii and must liave means of protecting themselves 

 or offsetting losses in the struggle for existence. Such 

 adaptations take a variety of form such as body- 

 armor, concealing coloration, attack weapons, or be- 

 havior ])atterns of escajie. High rates of mortality are 

 oH'set by high rates of reproduction or, in some lower 

 organisms, by considerable power of regenerating 

 wiiole organisms from fragmented parts. 



The manner in which reproduction occurs and tlie 

 special structures concerned with rejiroduction vary 

 with each type of animal and often with each indi- 

 vidual species. These adaptations are universal and 

 too numerous even to attempt to classify at this point 

 but are certainly obvious to all. Tlie primary objec- 

 tives in the life of each species are to maintain the 

 e.xistence of the individual and to reproduce its own 

 kind, and all adaptations to live in favorable habitats 

 are designed toward these ends. 



Maliiral sclrrtion 



To be heritable, a variation must have been 

 caused either by mutations in the genes or chromo- 

 somes of the individual or by new combination of 

 genes. Mutations are produced at random, and 

 mostly independent of natural environmental condi- 

 tions, although there is some experimental evidence 

 that they may be induced or increased in frequency 

 by cosmic rays, ultraviolet rays, heat, and certain 

 chemicals. There is no reason to believe, however, 

 that environmental factors can ordinarily influence 

 the kind of mutations that occur. 



Heritable variations in the structure of organisms, 

 and in their physiology and behavior as well, may be 

 favorable, unfavorable, or of neutral value to the ex- 

 istence of the species. Variations that decrease the 

 efificiency of a species in its struggle for existence 

 against competitors and unfavorable environmental 

 conditions usually disappear, but variations that in- 

 crease this efificiency give those individuals that pos- 

 sess them a better chance for survival and for giving 

 birth to similar offspring. Thus, there is natural se- 

 lection of the fitter individuals, and a gradual im- 

 provement in the relations between the species and 

 its environment. It is in this way that adaptations 

 are established. A better understanding of the eco- 

 logical relations between different species and be- 

 tween species and the environment will contribute to 

 a better understanding of the process of evolution. 

 At the same time a thorough understanding of the 

 processes of evolution is necessary to understand how 

 organisms become adapted to live in particular habi- 

 tats (Simpson 1953). 



A close study of differences between individuals 

 shows that within many species convergent evolution 



occurs under similar environmental conditions. 

 Many of these variations an- genetic and apparently 

 due to natural selection. The i)est established corre- 

 lations are the following, aithougii even they are sub- 

 ject to fre(|uent exceptions (Mayr 1942. Uobzhansky 

 1*)51 ) : 



I^ergmann's rule: geographic races of a species 

 possessing smaller body-size are found in the warmer 

 parts of the range, races of large body-size in the 

 cooler parts. This appears true for cold-blooded as 

 well as warm-blooded animals (Ray 19()0). 



.\i.i.i;.\"s kii.e: tails, ears, bills, and other ex- 

 tremities of animals are relatively shorter in the 

 cooler parts of a species' range than in the warmer 

 l>arts. 



Gloger's rule: in warm-blooded species, black 

 pigments increase in warm and humid habitats, reds 

 and yellow-browns prevail in arid climates, and pig- 

 ments become generally reduced in cold regions. 



Races of birds in the cooler parts of a species' 

 range lay more eggs per clutch than races in the 

 warmer parts of the range. Likewise the number of 

 young per litter of mammals averages higher in cooler 

 climates. 



The stomachs, intestines, and caeca of birds that 

 live on a mixed diet are relatively smaller in the 

 tropical- than in the temperate-zone races of a species. 



The wings of birds that live in a cold climate or 

 in high mountains are relatively longer than those of 

 close relatives that live in lowlands or in a warm cli- 

 mate. 



Races of birds in cool climates are more often and 

 more strongly migratory than races in warm climates. 



Races of mammals in warm climates have less 

 under-fur and shorter contour hairs. 



Fish of cool waters tend to have a larger number 

 of vertebrae than those living in warm waters. In- 

 crease in salinity tends to induce the same result as 

 low temperature. 



Fish that inhabit swift waters tend to be larger 

 and more streamlined than inhabitants of sluggish 

 or stagnant waters. 



Cyprinid fishes, isolated in desert springs, tend to 

 lose their pelvic fins. 



Land snails reach their greatest size in the area 

 of optimum climate within the range of the species. 



The relative weight of snail shells is highest in 

 the forms exposed to the highest radiation of the sun 

 or to the greatest aridity. 



Land snails tend to have smooth, glassy, brown 

 shells in cold climates, and to have white or strongly 

 sculptured shells in hot dry climates. 



It would appear at first glance that several of 

 these rules have a physiological basis: for instance, 



The general nature of responses 9 



