260 SECRETS OF ANIMAL LIFE 
glimpses. Thus the very frequent recurrence of a 
logarithmic spiral—in shell and horn, in intestine 
and cochlea—probably expresses a deep-seated 
growth-necessity. (2) Many an organ that looks 
at first sight complicated, say the pancreas or 
sweetbread, consists of the endless repetition of 
the same little structure-unit, consisting of groups 
of cells and the like. Some of these structure-units 
are very definite and have probably formed part of 
the common inheritance of more than one great 
series of animals, now attaining prominent expres- 
sion and again sinking into insignificance, but never 
lost from the racial currency. Thus it may well be 
that the structural units of the sensory lateral line of 
most fishes are really the same as the lateral sense- 
organs of certain marine worms known as Capitel- 
lids. “In both cases,” Professor Willey writes, 
“the essential organs consist of small, solid, round- 
ish, epidermal buds, from which stiff sense-hairs 
project freely into the surrounding medium; and the 
resemblance is further enhanced by their segmental 
arrangement. The correspondence could hardly be 
greater, the convergence could hardly be closer, 
the homology could not be more remote than 
infinity.” It may be, however, that we have here 
to do not with Nature repeating herself, but with 
the conservative persistence of a well-defined 
structure-unit, or “ morphon,” as it has been called. 
(3) Every art is limited by its medium, and so is 
organic evolution. We must not think of an animal 
having carte blanche in its morphogenic speculations. 
