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 44 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 44 concentric laminee 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, 44 unfortunately for the theory of the pearls 
being formed to repair the shell, they are often found imbedded in the flesh of 
the animalit 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 Desuayes. The 
loose pearls, he observes, 44 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 calculi , 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. 
Linnaean 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. III.-NO. XXIV. 3 S 
