NATURAL SELECTION 387 



to conditions within the body during development. Thc}^ include 

 adaptations of the spc^cies, but arc sometimes non-adaptive in so 

 far as external conditions are concerned. These two arc ordinary- 

 fluctuating variations which appear independently of external 

 conditions, and mutations. 



Both fluctuating variations and mutations are available as 

 materials for natural selection, since they are known to be heritable 

 as they occur. The former differ from the latter in that they may 

 arise, and probably do arise in most cases, from the recombination 

 of unit characters made possible by sexual reproduction, but in 

 that they are both the direct result of chromosomal determiners 

 and inherited bodily qualities, both are definitely of the heritage. 



Importance of Variations to the Individual. A further fac- 

 tor emphasized by Darwin in the Origin of Species is value to 

 the individual. Since no indifferent character can be of vital 

 importance to an individual, only useful or harmful characters can 

 play an active part in the shaping of a species. Indifferent char- 

 acters may, however, be incidentally or accidentally selected. 



Among the indifferent characters of animals may be included 

 slight variations in the color and pattern of insects. Some butter- 

 flies have brightly colored ocelli, or eye-like spots, on the hind 

 wings which vary in number although not ordinarily to a sufficient 

 extent to cause a distinct difference in appearance, since a row of 

 spots is usually present. In the human race, however, variations 

 in resistance to or tolerance of the typhoid bacillus is of primary 

 importance. Some individuals suffer no ill effects from the presence 

 of these bacilli, and become carriers, while to others the disease 

 caused by their presence is fatal. A race of carriers would be, in. 

 effect, immune from the disease, while an ordinary population is 

 very variable in susceptibility. 



Segregation. The last condition necessary for natural selection 

 is something to separate the individuals of a species into groups 

 possessing different variations or to preserve only a limited portion 

 of the individuals. The cause of segregation may be mere spatial 

 isolation brought about by topographic or climatic change or 

 migration, or it may be the relationships of organisms within a 

 limited region. The two causes are closely linked with distribution 

 and adaptive radiation respectively. 



Spatial isolation may have varied effects upon the organisms 

 that come under its influence. It was made the basis of a theory 



