116 EVOLUTION AND GENETICS 



surface, are able to live. Some species live under extremely limited 

 favorable conditions, like the myrmecophilous insects; others 

 seemingly thrive almost without restriction, like the ubiquitous 

 English sparrow. If the characters of these species are such as 

 actually to enable them to live only if the environment is favorable 

 within narrow limits, they are said to be specialized; if such that 

 they can live equally well under any of a variety of conditions, they 

 are called generalized species. Fitness for life under definite con- 

 ditions of all degrees is known as adaptation. 



Variability of Environment. The second factor in existence, 

 environment, is no less variable. From day to day and from sea- 

 son to season conditions of temperature, light and moisture change, 

 and with them come changes in the food supply. Change in envi- 

 ronment demands a change in the organism, hence mammals 

 acquire thicker fur in the winter, and certain aquatic snails seal 

 their shells with a mucus plug and lie dormant when drought robs 

 them of their normal habitat. However, many of the more highly 

 developed animals are able to exist in spite of extreme fluctuations, 

 and to remain in the same locality in spite of seasonal or other 

 changes. 



The Time Factor. Although environmental change, or exposure 

 to different environments, may result in different responses among 

 the individuals of a given species, it has been found impossible 

 until recently to produce such changes experimentally as a per- 

 manent modification of the species. Midler has succeeded in 

 producing permanent modifications of fruit flies by subjecting 

 adults to the action of X-rays. After this treatment mutations 

 appeared in the progeny of the flies much more abundantly than 

 in control cultures which had not been subjected to the rays. The 

 results depended upon the length of treatment with X-rays, and 

 in the most heavily treated lots of flies reached a maximum of 150 

 times the frequency of mutation in the controls. While these 

 results are interesting and significant, especially in the field of 

 genetics, they do not refute the fact that the manipulation of con- 

 ditions such as occur in a natural environment has not yet pro- 

 duced permanent modifications. We must remember in judging 

 experiments of the latter type, however, that even a factor operat- 

 ing since the beginning of recorded history would have had rela- 

 tively little time to effect a permanent change in comparison to 

 the vast periods which have come and gone during the existence 



