CHAPTER OF CRITICISM. 485 



I see any thing opposed to the validity of this theory in what Mr. Levison has 

 advanced. In the first place, Mr. Levison, in his remarks, has not fully stated, 

 or has misapprehended, the view 1 gave in the lecture, of the formation of pearl. 

 I said, that in all cases it was looked upon as the result of secretion depending 

 upon irritation, produced either by a foreign substance — as a particle of sand — 

 being introduced into the shell, or the attacks of some of the boring Annelides. 

 (see p. 408). Now the sections that Mr. Levison has made u in order to ascer- 

 tain the modus operandi" of Nature in the formation of pearl, confirm this 

 view. If they are formed by a secretion from the mantle, they must present a 

 series of " concentric lamince beginning with a small nucleus, and enlarging, each 

 successive layer, like a number of watch-glasses, the smallest, being the central 

 one." Mr. Levison states that, " unfortunately for the theory of the pearls 

 being formed to repair the shell, they are often found imbedded in the flesh of 

 the animal ;•' it is not, however, asserted that this is the only mode of forming 

 pearls, but that they may be formed around any foreign substance. Still in this 

 case there is some difficulty in accounting for such a situation of the pearl ; and 

 I will give the explanation of the phenomenon in the words of Demiayes. The 

 loose pearls, he observes, " are met with more especially in the substance of the 

 adductor muscles ; now if it be remembered that these muscles shift their place 

 in proportion as the animal grows, it may readily enough be allowed that a 

 pediculated pearl, developed on the surface of the muscular impression itself, 

 might be detached from its connection with the shell, by the advance of the 

 muscle, become free in the substance of this muscle, and there continue to 

 increase with more or less rapidity." With regard to Mr. Levison's theory of 

 their being formed in a similar manner to calcidi, there can be no doubt that these 

 bodies are formed by the deposition of successive particles of matter, secreted from 

 the fluids of the animal in which they are found, in the same manner as calculi ; 

 and it is on this account that they must with those bodies be regarded as dis- 

 eased productions. 



I cannot, however, subscribe to Mr. Levison's supposition that these bodies are 

 formed by deposition from a solution of the nacreous portion of the shell. There 

 is no evidence to give the shadow of a probability to such a theory. All analogy 

 in the functions of the Animal Kingdom are opposed to it ; and the physical 

 principle on which he builds his hypothesis can never take place in the vital 

 economy. 



LINN.EAN AND NATURAL SYSTEMS OF BOTANY. 



I must now beg a few lines for your intelligent correspondent Mr. Edwin Lees, 

 who opened his letter (p. 380) with so warlike an aspect, that I began to tremble 

 for the consequences. However, his kindly feelings appear to have prevailed, 



vol. in. — no. xxiv. 3 s 



