COMMON VIPER. 67 
closely as almost to appear like a solid mass; but no 
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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