

XXIII.] THE CAUSES OF DEVELOPMENT. 305 



One of the most important laws to be borne in mind in Organs 

 all speculations of this class, is that law in virtue of which ^^liTse- 

 every organ improves with use. This indeed is a parti- the diffi- 

 cular case of the law of self-adaptation. Exercise, pro- first 

 vided that it is not beyond the limit of what is good for oiig"i- 

 health, increases the strength of a muscle or the sensitive- 

 ness of a nerve. The difficulty is not to account for 

 almost indefinite improvement in an organ, but to accoxmt 

 for its origin. I cannot make any suggestion whatever as 

 to the possible origin of nerve, but perhaps it is not absurd Origin oi 

 to think that the fibrous structure of muscle was originated muscle: 

 by the sarcode substance of the earliest structureless 

 though living beings acquiring the habit of contracting in 

 one direction more readily than in any other. We know 

 that the sarcode has contractile power in organisms which 

 have no visible structure, and the simplest rudimentary 

 muscular structure consists in a fibrillation of the sarcode.^ 



I have now stated my belief that some of the simpler 

 structures belonging to the vegetative system have pro- 

 bably been produced by the action of inorganic forces on 

 the organism ; and that muscular structure may possibly, 

 though I do not say probably, have been produced by the 

 action of the organism itself in response to impressions 

 from without. Of course these two factors are always 

 both present, though acting in very unequal proportions in 

 different cases. But there are structures for the origin of 

 which it is, I believe, utterly impossible to account by any 

 such merely physical theory; and which can only be referred 

 to an organizing intelligence. I refer to such organs as of the eye 

 the eye and the ear. If it is certain, as I think it is, that ^"^ 

 the flow of the nutritive fluids through cellular tissue, for 

 successive generations, must have a tendency to form a 

 rudimentary circulating apparatus, it is at least equally 

 obvious that the action of light falling on the eye for any 

 number of generations can have no similar tendency to 

 produce the optical apparatus of the eye. Nor can the 



attempts to account for the one without the other. It would be different 

 if the ganglia were an outgroivth of the fibres, as the brain is of the 

 spinal cord. 



''■ Dr. Wyville Thomson on the Embryology of the Echinodermata, Natural 

 History Review, October 1864. 



X 



