M. C. Mereschkowsky on the Hydroida. 243 
considerable, as we have already seen, about ten or even 
more ; and, in fact, it is most usual for their number to ex- 
ceed four or five. 
Fig. 4. 

In these cases, and especially where we have to do with 
about ten metameres, the explanation of the origin of these 
articulated forms which I have just given cannot suffice ; 
and hence it must be supplemented by a very in- 
teresting law, which Prof. N. Wagner of St. Peters- 
burg has denominated the law of physiological inertia, 
and has so happily applied to the explanation of the 
incredible number of metameres (articulations) with 
which various worms (Annelida) are furnished. 
According to this law, some cause having origi- 
nated two or three metameres, the appearance of the 
following metameres may be brought about without 
the further aid of the primary cause, but solely under 
the influence of a tendency that the organism has to repeat 
the process of the appearance of metameres (a process at first 
induced by some external influence [chogue], such as abun- 
dance of food) by cnertia, as it were, until finally resistance, 
under different forms, may put a stop to it. 
The different qualities and properties of an organism are 
often retained, by force of heredity, without interruption and 
without modification during a long series of generations, 
even when the cause which has induced these qualities has 
long disappeared. It is so in the case in question: a certain 

