THE EXTENSION OF THE PRINCIPLE. 109 
to animals and to man ; an application far out- 
weighing in importance all that has hitherto 
been effected. In a letter addressed to Sir W. J. 
Hooker, and published in the <£ Companion to 
the Botanical Magazine,” for May 1836, I thus 
wrote — “ I feel well-assured that this method 
of importing plants would likewise he extremely 
useful in the introduction of many of the lower 
hut most interesting tribes of animals, which have 
never yet been seen alive in this country.” In 
April 1838, in a lecture delivered by Professor 
Faraday,* on the closed cases, at the Royal Insti- 
tution ; and later in the same year at the meeting 
of the British Association at Liverpool, I ex- 
pressed my opinion, that animals and man might 
benefit by the same plan of treatment which had 
proved so successful with plants. In 1842, in 
the first edition of this work, I stated, that a 
little reflection will convince us that this idea is 
not so visionary as it might appear at first sight, 
as I had proved by numerous and long-con- 
tinued experiments that the air of London, if 
duly sifted, was perfectly fitted for the respiration 
of all plants, even of those with the most deli- 
cate leaves, such as the Trichomanes speciosum, 
which may, in fact, be considered as a test-plant, 
as regards the purity of the air. Now this same 
* Vide Letter of Prof. Faraday, in the Appendix. 
