UIE AMERICAN NATURALIST [Vol. LIII 



of the University of Illinois from Alamogordo, New 

 Mexico, and placed in a sand-bottomed wire screen cage. 

 On July 7, between 11 a.m. and 1 p.m., twenty-three eggs 

 were deposited in the sand on the bottom of the cage. 

 The eggs were about 1 cm. in length, ovoid in shape, and 

 covered with a grayish-white shell of leathery texture. 

 Some were opened and found to contain living embryos 

 of about 2 mm. length. Several times thereafter, during 

 a period of two weeks, eggs were found in the cage, always 

 lots of about twenty. The deposition of the eggs was 

 never observed. None of the eggs hatched, although liv- 

 ing embryos were found in eggs opened a week after 

 deposition. Such embryos were about 6 mm. in length. 

 P. douglassii has not been observed to lay eggs, although 

 a few eggs of P. modestum were discovered in the cage 

 in which these animals were kept. These were found in 

 small numbers only and differed from those just de- 

 scribed in being light yellow in color and having no 

 leathery shell. They were probably abortive. As the 

 observations of Watson and Ditmars appear to be well 

 founded, it is possible that the genus is divided with re- 

 spect to the retention or deposition of the eggs, or that 

 in the same species different conditions may alter the 

 length of time the egg is retained in the maternal body, 

 as is the case among the adders. 



II. Envieonmental Factoks 

 As has been concluded (1917a), it is dangerous to as- 

 cribe to any one factor or group of factors the supreme 

 role in determining the seasonal or general distribution of 

 a species. These factors are certainly not the same for 

 all species even in the same environment, and before defi- 

 nite conclusions can be drawn a careful analysis of the 

 habitat must be made, and experimental data must be ob- 

 tained as to the reactions of the animals in gradients in- 

 volving the factors capable of variation. Unfortunately, 

 it is not possible or practicable to construct effective 



