90 
ON THE PHYSIOaiS'OMY OE SERPEXTS. 
the terrestrial species devour mammifera and birds useful 
to man, and very often destroy the nests to devour the 
eggs or the young. 
PROPAOATIOH. 
In our climates, where serpents only produced young once 
a year, copulation takes place most frequently in the first fair 
days of April or May. For this act the two sexes entwine 
their bodies together, so as to seem only a single individual 
with two heads looking face to face ; the male then intro- 
duces into the female cloaca the two cylindric bodies 
covered with spines, which on being turned inside out are 
drawn from under the tail : the two sexes remain thus united 
for several hours but we are unable precisely to fix the 
duration of their copulation. It is, at least in our indigenous 
species, a space of three or four months, before the eggs 
are ready to be laid ; during this interval they undergo a 
species of incubation in the belly of the mother ; for on 
opening the eggs just after they are laid, we almost always 
perceive a foetus more or less developed, and sometimes 
even perfectly formed. In this latter case, the young are 
shut up in a thin membrane, which they tear at the moment 
of birth to commence their independent existence. In a 
great number of serpents, on the other hand, the eggs are 
enveloped in a very tenacious tunic of a coriaceous nature, 
or rather resembling parchment ; the young, being only 
imperfectly formed when the eggs are laid, they require 
sometimes the space of a month more before the hatching 
is accomplished. On this depends the distinction which 
has been made between viviparous and oviparous serpents : 
a distinction which, indeed, is not founded on any other 
ground than a greater or less development of the foetus in 
the egg at the time of laying, or on the nature of the ex- 
terior covering of the egg. Ophidians are really always ovi- 
parous, and it is wrong to compare this species of genera- 
tion to that of the mammifera, where the young receives 
its nutriment through the medium of the placenta. 
^ Lenz, p. 52. 
