408 
Proceedings of Royal Society of Edinburgh. 
on two slides : the period was normal. After separation each of 
the ex- conjugates divided once : on the third day they died off. 
In anticipation of something of this sort from analogy in higher 
forms, I intended to let the two pairs run their natural course, fore- 
going the desire to examine their nuclear condition. In view, 
therefore, of the incompleteness of the experiment, it is perhaps 
unwarrantable to draw any results regarding hybridisation and 
infertility, or even the “fixity of species,” so far down in the 
animal scale. 
As has been previously mentioned, numbers of observers have 
remarked that the comparative regularity with which binary 
fission proceeds under favourable circumstances decreases as the 
time increases since the last conjugation, and one has often 
wondered if it would not he possible to express this decrease by 
means of a mathematical curve or formula. In this connection, it 
is Maupas’ chief distinction to have established that in the case of 
each species this power of binary fission comes to an end after a 
definite number of divisions ; and that, were there no other method 
of restoring this potentiality to the individual, the species would come 
to an end. With the later stages of this gradual loss of fission- 
energy, he found distinct degeneration of the creature associated. 
In this degeneration he distinguished two well-marked stages. 
The first stage is not accentuated by any particular external change 
in the infusorian, unless, possibly, a slight reduction in size.* It 
continues to feed and multiply in the normal manner, but all the 
while it is giving rise to successors that are entering the second 
stage of degeneration. Moreover, when preserved in the ordinary 
method it is found to have undergone a certain atrophy of its 
nuclear apparatus. The macronucleus fragments (Styl. pust.), or 
may disappear altogether (Styl. myt.). The micronuclei are 
reduced to one, or even none (Styl. pust. and Oxytricha sp.). On 
the other hand, after such reduction they may later increase to 
numbers in excess of the norm (Styl. myt. and Onychodromus gr.). 
In the second stage of this senile degeneration the infusorian loses 
its power of multiplication. It no longer takes in food, and its 
body in consequence becomes quite clear. There is now a marked 
decrease in size,! and atrophy of external organs and appendages 
* In the case of Stylonichia pust. this redaction varied from 25 to 50 y. 
+ Stylonichia pust. now measures 70-90 y in place of the normal 160 y. 
