288 ON THE PROGRESSIVE DEVELOPMENT OF 



molecules, namely, spherical or spheroidal. But there is this 

 essential difference. The phenomena of the mineral molecules 

 are best explained upon the supposition that they are homogene- 

 ous, or of like nature throughout ; whereas, as far as our researches 

 go, the ultimate forms of organization are not homogeneous, 

 but consist of amembranous or filmy envelope of extreme tenuity, 

 inclosing within its cavity a transparent fluid, constituting, in 

 fact, a close spheroidal cell or vesicle. 



It is not necessary for our present purpose to attempt to define 

 the difference between the simple monads of animal, and the sim- 

 ple vesicles of vegetable existence; and when it is stated that of 

 the former five hundred millions have been crowded into the 

 space of one cubic inch without apparently interfering with each 

 other, and that of the latter still greater numbers have been 

 observed to occupy the same space, it will be seen that the very 

 minuteness of these bodies in itself opposes a barrier to our inves- 

 tigations, the bounds of which we cannot pass. The subject 

 which more immediately demands our attention is the gradual 

 development of the mysterious principle of life, as shown in the 

 variations of organic structure throughout the scale of vegetable 

 being, commencing with the most simple vesicle or germ of vege- 

 table existence, and tracing the increasing development of the 

 vegetable structure till we arrive at the most complex and 

 elaborate of its forms. 



The most simple form of vegetable existence, according to the 

 views here expressed, of which we can form a conception, is a 

 close cell or vesicle — a simple vesicle, consisting of a membranous 

 film of extreme tenuity enveloping and inclosing within its central 

 cavity some fluid, or perhaps aeriform matter. Such a vesicle 

 may be observed in the granules of the Lepraria viridis, that 

 green powdery incrustation which is found in such abundance on 

 trees, old posts or rails, walls, &c. in damp shady situations. 

 This green crust is in fact one of the most simple forms of 

 vegetable being with which we are acquainted, and when ex- 

 amined with the aid of a powerful microscope, will be found to 

 consist of innumerable small granules* of a spheroidal shape, 

 very loosely connected together in fours. 



It is not meant to be asserted that these granules, cells, or 

 vesicles are perfectly simple — that they are the ultimate organic 

 elements of the plant ; for it is not at all probable that we should 

 here have arrived at the real elementary composition. On the 

 contrary, there is much reason to believe that the external filmy 

 coating of these vesicles, though of such extreme tenuity, is itself 

 compounded of numerous other cells — that it in fact consists of 

 a congeries of vesicles of inconceivable minuteness so united 

 together as to form a continuous surface ; each of which secondary 



* That these granules are vesicular is evident, for by touching them with the 

 point of a fine needle, when viewed in water under the microscope, their filmy coats 

 will be ruptured and will be observed to present irregular torn edges. 



