CHAIN OF CYCLOSALPA AFFINIS 397 
must hold that unless the new correspondence includes some- 
where what we hold as one or more ‘causal factors’ not much has 
been accomplished. If, for example, we extend the inquiry to the 
question of the dependence of the size scheme of the zooids upon 
growth and other internal factors on the one hand, and upon en- 
vironmental factors on the other; and if here, too, we find further 
correspondence, our belief in the essential identity as we might 
say, unless standing for extreme exactness of expression, would 
be reached. 
We shall see that the size scheme of the zooids in the wheels is 
almost certainly foreshadowed before the wheels themselves are. 
If this be so, then the resemblance between the Cyclosalpa chain 
and the Salpa chain is considerably closer before than after the 
wheels appear in the former. But it is difficult, if not impossible, 
to attribute the block production in the Salpa chain entirely to 
other than inherent factors, of which growth seems to be the most 
immediate. So far, therefore, as we can rely upon our evidence 
for the marking off of the Cyclosalpa chain into groups before the 
wheels are formed we seem to have placed the notion of corre- 
spondence between the wheels and the blocks on firm ground. 
Evidence still more convincing perhaps, that the wheels and 
blocks correspond in a strict biological sense, in the sense that both 
are expressions of periodicity in growth, is found in the fact that 
growth and development are observed to be periodic in so wide 
a range of living beings. The growth of plants for example, ap- 
pears to be nearly if not quite always of that nature. Finally, 
belief in the correspondence would, so far as we can see, reach 
high water mark, should it be finally made very probable that not 
only growth and development but all strictly biological processes 
whatever, are periodic. We are undoubtedly a long way from this 
last conception. Certain it is, though, that we now have sufficient 
facts to make the hypothesis of periodicity as warrantable as its 
opposite, namely, that certain phenomena are continuous in the 
sense of not being automatically interruptive and group-wise. 
We may now proceed to the handling of our data. 
