record which has been checked frequently by eggs placed in 

 an artificial incubator, where they require sixty days to 

 hatch. The measuring of the true length of incubation 

 amongst other reptiles becomes more and more perplexing 

 as one takes up the succeeding ascending orders. It is ex- 

 ceedingly difficult to decide what is the true length of incu- 

 bation with many snakes and lizards because of the ease 

 with which the embryonic development of these animals 

 can be slowed down, or temporarily suspended, and because 

 of the high probability that the same reptile species may 

 exhibit all gradations between complete oviparity and ovovi- 

 parity. gradations which can cause a difference of days or 

 even weeks in the apparent length of incubation in the same 

 species. This possibility also is mentioned here because it 

 •is probable that oviparity and ovoviparity are interchang- 

 able according to environment, in certain other species. 



This causes a wide variation in the records of incubation 

 lengths of reptiles, and adds to the difficulty of reaching con- 

 clusions on the point now in mind, yet in spite of this and the 

 paucity of records, it seems to me that reptiles also exhibit a 

 tendency to shorter incubation periods as the species is 

 higher in its scale of life, which tendency seems to show in 

 the data given in Table No. 7. 



TABLE NO. 7 

 Incubation Periods of Reptiles 



Period Authority 



Green Turtles Si/o weeks 119 



Loggerhead Turtles 8I/2 weeks 119 



European Tortoise 20 to 44 weeks 120 



Tuatara Lizard 52 weeks 121 



Crocodile 12 weeks 120 



Alligator (in incubator) 8I/2 weeks 122 



Common Swift 6 to 8 weeks 123 



Python reticulatus 6 to 8 weeks 125 



Python molurus 10 weeks 120 



Black Snake 81/, weeks 123 



Fox Snake 7 3/7 to 8 6/7 weeks 123 



Corn Snake 6 to 8 weeks 123 



Yellow Rat-snake 11 weeks 123 



Ring Snake 5 6/7 weeks 123 



Milk Snake 8 1/7 weeks 123 



King Snake 6 to 8 weeks 123 



Coral Snake 8 2/7 to 8 4/7 weeks 123 



If reptile eggs vary in incubation lengths according to the 

 applied temperatures, it is proper to ask if there be any evi- 

 dence in the temperature of reptiles, showing trends similar 

 to those in birds ; reptiles are said to be "cold-blooded," but 

 it can be shown that this term is only relative. The average 



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