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ORR. [Vol. I. 



relatively in a new position, organs which had arisen in their 

 old position through a long course of phyllogenetic develop- 

 ment. It seems, also, that peculiar changes of environment 

 must be supposed in order to have made this sudden change of 

 position permanent. Again, it might possibly be supposed that 

 the ancestral worm gradually became insensible to, and unaf- 

 fected by, the force of gravity ; so that in motion and rest the 

 morphological axes of his body bore no relation to the direc- 

 tion of gravity. But the influence of gravity on animals is so 

 universal, that on the last supposition we are almost forced to 

 imagine a temporary suspension of gravity. Perhaps the greatest 

 objection to the annelid-inversion theory, no matter what method 

 the inversion may be supposed to have followed, is the absence 

 of intermediate forms, or traces of them. The most plausible 

 methods — those of gradual, lateral, or longitudinal inversion, or 

 gradual shifting of the organs — seem the most hkely to have 

 left intermediate forms or traces of them. 



The theory of the dorsal approximation of lateral nerves has 

 the advantage of being less specialized in its application. It is 

 not dependent on such difficult tasks as showing how the 

 oesophagus passed through the brain, or how a worm-like form 

 became inverted. It can also refer to a general intermediate 

 form. As a rule, those biological theories which are based on 

 a few far-reaching facts, and allow greater scope for processes 

 of \vhich we are ignorant, but which are from analogies imagi- 

 nable, are more trustworthy than those theories having perhaps 

 equivalent facts, but which are yet absolutely dependent on 

 detailed complicated processes of which we are equally ignorant, 

 and which are less easily imaginable. The annelid theory has 

 been skilfully adapted to explain certain particular details of 

 vertebrate anatomy, — for instance, a relation of the vermicular 

 locomotory appendages with setae, to the paired and unpaired 

 fins with their rays and spines. Such explanations give the 

 theory a false appearance of strength ; but it must be remem- 

 bered that, until the main stumbling-blocks of the theory are 

 cleared away, such explanations are comparatively valueless 

 as support for the theory, and add almost nothing to our 

 knowledge of the probabilities. The special application of the 

 dorso-latcral nerve-theory has been attempted in no such degree 

 as the above instance. Its possibilities of explanation remain 



