COMMON VIPER. 67 



closely as almost to appear like a solid mass ; but do 

 sooner is it emancipated than it assumes all the activity and 

 virulence which belong' to the species. If a female Viper 

 about to bring forth her young be killed, and the young 



ones set at liberty by opening the abdomen, they will im- 

 mediately crawl about, and on being irritated will throw 

 themselves into an attitude of defence. 



A certain degree of temperature, varying in the different 

 classes, is requisite for the development of the embryo. 

 In the mammalia this temperature is afforded by the 

 standard heat of the body of the parent, within which the 

 young is brought to perfection ; in birds it is also, with 

 few exceptions, communicated by the parent, but by 

 means of incubation, or sitting on the eggs after they are 

 excluded from the body ; in the reptilia, on the contrary, 

 it is communicated solely by the surrounding atmosphere ; 

 and this is equally the case whether the eggs are deposited 

 before the growth of the embryo has commenced, or the 

 young are perfected within the mother, and brought forth 

 alive. In the latter case, as I have already observed, 

 when speaking on the development of the young of the 

 common Viviparous Lizard, Zootoca vivipara, it is by the 

 exposure of the body of the mother to the sun's rays that 

 the necessary degree of heat is obtained ; and hence during 

 the period of gestation it is very common to see the preg- 

 nant female Viper lying, as it were, all abroad, with flat- 

 tened body, basking in the sunshine ; and at this time she 



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