OB f GIN OF THE ANIMATED TRIBES. 



91 



ken, a foliated texture. The examination of it has even 

 thrown some light on the character and mode of forma- 

 tion of natural shell. "The plates into which the sub- 

 stance is divisible have been formed in succession, and 

 certain intervals of time have elapsed between their for- 

 mation; in general, every two contiguous laminae are 

 separated by a thin iridescent film, varying from three to 

 the fifty millionth part of an inch in thickness, and pro- 

 ducing all the various colors of thin plates, which corres- 

 pond to intermediate thicknesses : between some of the 

 laminae no such film exists, probably in consequence of 

 the interval of time between their formation being too 

 short ; and between others, the film has been formed of 

 unequal thickness. There can be no doubt that these 

 iridescent films are formed when the dash-wheel is at rest 

 during the night, and that when no film exists between 

 two laminae, an interval too short for its formation (arising, 

 perhaps, from the stopping of the work during the day) 

 has elapsed during the drying or induration of one lamina 

 and the deposition of another."* From this it has been 

 deduced, by a patient investigation, that those colors of 

 mother-of-pearl, which are incommunicable to wax, arise 

 from iridescent films deposited between the laminae of its 

 structure ; and it is hence inferred that the animal, like 

 the wheel, rests periodically from its labors in forming 

 the natural substance. 



These, it will be owned, are curious and not irrelevant 

 facts ; but it will be asked what actual experience says 

 respecting the origination of life. Are there, it will be 

 said, any authentic instances of either plants or animals, 

 of however humble and simple a kind, having come into 

 existence otherwise than in the ordinary way of genera- 

 tion, since the time of which geology forms the record ? 

 It may be answered that the negative of this question 

 could not be by any means formidable to the doctrine ot 

 law-creation, seeing that the conditions necessary for the 

 operation of the supposed life-creating laws may not have 

 existed within record to any great extent. On the other 

 hand, as we see the physical laws of early times still 

 acting with more or less force, it might not be unreason- 

 able to expect that we should still see some remnants, or 

 partial and occasional workings of the life-creating energy 



* Mr. Leonard Hornorand Sir David Brewster, on a substance 

 resembling shell.— Philosophical Transactions , 1S36. 



