1 86 THE METHOD OF DARWIN. 



such relations existed. The whole problem has 

 advanced much farther toward a solution on the 

 morphological than on the physiological side; 

 but, as might have been expected, the demon- 

 stration of the exact relations between the 

 various elements of muscle-fibre and those of 

 the electric organs has enabled Engelmann to 

 indicate some very important and fruitful lines 

 for physiological experiment. 



Although the homology of cross-striped mus- 

 cle-fibre and the constituents of the electric 

 organ has been proved even to many histo- 

 logical details, and the morphological changes 

 by which the former has been transformed into 

 the latter have become quite clear, the diffi- 

 culty of explaining the development of electric 

 organs from muscular tissue by means of natural 

 selection is as great as ever. Engelmann has 

 significantly suggested the existence of func- 

 tions that we are entirely ignorant of; and 

 refers, by way of illustration, to the recent 

 remarkable discoveries of the effects of the 

 removal, partial and complete, and grafting of 

 the thyroid gland, 1 and to Brown-Sequard's 

 demonstration of the existence of an extremely 

 important "internal secretion" in the renal and 



1 Archives de Physiologic Normale et Pathologique, 1890- 

 1894, and elsewhere. 



