THE METHOD OF CREATION OF ORGANIC FORMS. 183 



modified ; and in the Mammalia the digestive and circulatory- 

 systems have both become unsymmetrical ; and the cranium, even, 

 in the Cetacea. 



If evolution be true, the unsymmetrical forms have descended 

 from the symmetrical, and the asymmetry being thus not inherited, 

 is the result of laws which have interfered with the original tend- 

 ency to bilateral repetition. 



Many cases of bilaterally symmetrical diseases have been enu- 

 merated by physiologists, and I will select as an example one which 

 has come under my observation. They were those of two boys who 

 had had that disease involving the muco-dermal system called vari- 

 cella, while the crowns of the successional incisor teeth were still 

 inclosed in the mucous capsules of the alveolar walls. The deposit 

 of phosphate of lime forming their surfaces was interrupted by the 

 disease of the tissue, and the result was a surface pitted, or sculpt- 

 ured intaglio fashion. The sculpture of the two incisors of the 

 right side was precisely imitated by those of the left in reversed 

 order, even in minute details, which were numerous, thus produc- 

 ing a result not displeasing to the eye. This has been observed on 

 two distinct occasions some years apart. 



Another interesting example of bilaterally symmetrical disease 

 is recorded in a paper on " A Case of Universal Hyperostosis, etc.," 

 by Drs. Mears, Keen, Allen, and Pepper.* They describe the skele- 

 ton of a boy of fourteen which displayed an extraordinarily exos- 

 tosed condition, the bones themselves remaining in the condition 

 known as osteoporosis. They describe the uniform repetition of 

 the abnormal growths of one side on the other in the following 

 language (p. 22) : 



" Comparing the two sides externally, not only is there no dif- 

 ference in the extent and character of the disease, but there is the 

 most remarkable symmetry of the corresponding diseased bones, 

 which may be traced even into details. The disease begins and 

 ends on both sides at corresponding points, it changes in character 

 from simple porosity to the growth of osteophytes at corresponding 

 points ; if, on one side, the posterior part of the bone is most dis- 

 eased, the same is true of the other side ; if the osteophyte growth 

 is continuous or interrupted on one bone (fibula, Fig. 18), it is so 

 on the opposite one ; if one is unusually diseased at a tendinous or 

 aponeurotic insertion, so is its mate ; if a groove or a variation in 



* Sec "Proceed. Amcr. Philos. Soc," 1870, p. 19. 



