IOWA ACADEMY OF SCIENCES. 51 



eral and the species would be totally exterminated if such 

 separation could be made to cover all points where the 

 species occurs. 



Other cases of extreme specialization are to be found 

 among those forms that have adapted themselves to desert 

 conditions and which can hardly be conceived as having 

 the possibility of survival if forced back into humid con- 

 ditions with competition with forms of life of more gen- 

 eral character. The cave animals cannot survive under 

 ordinary conditions of light and outer air. Subterranean 

 animals must have their peculiar environment or perish 

 and deep sea animals are totally inadequate to the more 

 general conditions prevailing at shore line or in the shal- 

 low reaches of the sea. Some of our domestic animals are 

 practically dependent on man, some species of flies depend 

 solely on their resemblance to certain bees to get entrance 

 to nests and stores of food, while certain ants which have 

 adopted slaveholding are said to be entirely unable to 

 carry on the ordinary duties of the colony but are dependent 

 upon their slaves for their very existence. So, too, some 

 species of insects are dependent on a particular food plant 

 and will perish without it. 



Specialization in such cases means a kind of limitation 

 and total unfitness for existence outside of certain condi- 

 tions and hence extinction if those conditions fail. Pos- 

 sibly we might call this a form of change in environment 

 but it must certainly rank as a special form of environ- 

 mental change. 



This differs essentially in the fact that such forms have 

 in a certain way selected the route along which they have 

 traveled and thus foredoomed themselves to extinction. 

 In every case we must assume that such extremely special- 

 ized forms have been derived from more normal or gener- 

 alized forms; parasitic from free forms; desert forms from 

 those occurring in more humid regions; cave forms from 

 those occurring above ground; deep sea from surface or 

 shallow water species, and so on, and that the occupation 

 of the particular niche in nature has been one of selection 



