232 2 NATURE AND EERE: 
=} Ae 
et. 
was a splendid period in the history of the sciences, at. 
which observation, proving the intuitions of. genius true, 
exhibited by instances so astonishing the composition of 
the organized individual to be such that every one of the 
living molecules that make it up has in itself a principle 
of activity and of individual development. Whatever cor- 
rections need to be applied to the way in which Buffon and 
Bonnet, after Leibnitz, have unfolded that doctrine, it re- 
mains, in its essential tenor, the starting-point of a rich 
evolution for biology, and the true expression of what is 
real. 
The experiments just mentioned have been often re- 
peated and ingeniously varied by naturalists. Little fresh- 
water worms, to which the name of planarzz has been given, 
have been a subject of study to several savants, among 
others to Draparnaud, Moquin-Tandon, and De Dugés. 
The latter cut in two a number of single specimens of 
the largest kind, either across or lengthwise, and he ob- 
served each fragment build itself anew, in twelve or fifteen 
days in winter, and four or five in summer, the head pro- 
ducing a sucker and a tail, and the tail a head and a sucker, 
and the piece in the middle sometimes keeping its sucker, 
sometimes losing it and again forming it, together with a 
head and a tail.. Immediately upon the cutting, the outside 
bulges up like a cushion, while the centre shows the pulp 
exposed, and on this centre part the first outlines of the 
renewed portions make their appearance. A single one 
divided thus gives birth to several new ones, the size of 
which, at first proportional to the dimensions of the frag- 
ment, very quickly grows up to that of the original whole. 
More lately, Vulpian cut off the tail of a young frog, still 
inclosed in the egg, and put it in water. This rudiment 
of a tail maintained its life there, and developed regularly, 
passing through all the phases of its embryonic existence. 
Having reached the condition of perfect organization, its 
