December 1st, 1896.] Proceedings. xvii. 



wonder, for there was vitality in it. It has risen, phoenix- 

 like, out of its own ashes, to live henceforth under the name 

 Potential Energy. So also the theory of Epigenesis, first 

 propounded by Aristotle and defended by Harvey, that an 

 organism grows by differentiation of a comparatively homo- 

 geneous germ into the parts and structures which are found 

 in the adult, was strongly opposed, if not supplanted for a 

 time, by the rival theory that all the organs are not only 

 potentially but actually present in the germ, and grow by 

 accretion. The latter has now disappeared for ever, accord- 

 ing to the best authorities, and the pendulum has swung 

 back, as it were, to the old idea again. 



By Obstructive errors are meant those which under the 

 shadow of a deservedly great name have retarded progress 

 for a time, such as the physical theories of Aristotle, the 

 Corpuscular Theory of Light, the Proportionality of Refrac- 

 tion and Dispersion, the Wernerian theories of Geology, and 

 so on. 



Lastly, Occasional errors are the obiter dicta of scientific 

 men, freaks of the imagination, or ill-considered deductions 

 from observation, which are soon exploded, and leave no 

 legacy to posterity except a feeling of amusement whenever 

 they are met with. Among these may be mentioned the 

 once-renowned Urschleim or Bathybius, by which an acute 

 intellect was for a time deceived ; the hot ice, unaccom- 

 panied, however, by " wonderous strange snow," as Shake- 

 speare puts it, which was sprung upon an astonished world 

 in 1880 ; and lastly, the misdirected efforts of an astronomer 

 to find, amongst other things, the dimensions of the solar 

 system, certain mathematical constants and relations, and 

 even a foreshadowing of our chaos of weights and measures, 

 in the length and breadth and height of the passages and 

 vaults of the Great Pyramid. 



