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. 



