DEVELOPMEXT OF TEE FORMS OF ANIMAL LIFE. 



485 



aanic life in the simpler forms in which it at first 

 appeared — a state of things which can only be 

 vaguely surmised, but in regard to which no ex- 

 act information can be obtained. Or, secondly, 

 evading the difficulty of strict cosmical evolution, 

 we might suppose that vital conditions may have 

 been coeval with the first existence of physical 

 and chemical properties in the rest of natural 

 bodies. But this hypothesis would be exposed 

 to the objection that, according to the cosmical 

 view generally held by physicists, the whole ma- 

 terials composing the earth have originally been 

 subject to incandescent heat. Nor is the difficul- 

 ty abolished, but only removed to a more remote 

 period, by the supposition of the transport of 

 germs from another planet or their introduction 

 by means of meteorites or meteoric dust ; for, be- 

 sides the objection arising from the circumstance 

 that these bodies must have been subjected to a 

 very high temperature, we should still have every- 

 thing to learn as to the way in which the germs 

 arose in the far-distant regions of space from 

 which they have been conveyed. 



The incompleteness of the geological record 

 leaves us in the dark as to the time at which the 

 first dawnings of life appeared in the lower strata 

 of the earth's surface. The most recent research- 

 es tend to carry the origin of life back to a much 

 earlier period than was at one time believed, and 

 if the famous Eozoon be admitted as evidence, 

 even into that of the Laurentian strata. But 

 even if doubts should prevail with regard to the 

 presence of definite organized forms in the older 

 sedimentary strata, the occurrence in them of 

 carbon in the form of graphite in large quantities 

 makes the previous existence of living organisms 

 at least possible, and it may be that the complete 

 metamorphosis which these rocks have under- 

 gone has entirely removed all definite traces of 

 organization. 



Nor have we the means from geological data 

 of determining whether the beings of the vege- 

 table or of the animal kingdom first made t^eir 

 appearance. If we adopt the view which has for 

 some time been entertained by physiologists that 

 animals are entirely dependent, directly or indi- 

 rectly, on plants for the material which consti- 

 tutes their living substance, and that plants, as 

 constructive agents, alone have the power to 

 bring together the elements of lifeless matter 

 from such states as carbonic acid, water, and am- 

 monia, into the condition of the living solid, the 

 inference would be inevitable, at least for the 

 great majority of the animal creation, that they 

 must have been preceded by plants. But palae- 



ontology is as yet silent on this interesting ques- 

 tion ; and, if we consider the remarkable ap- 

 proach which is made in structure and proper- 

 ties between the lowest and simplest members of 

 the two kingdoms of organic Nature, so that at 

 last all distinction between them seems entirely 

 to vanish, and a set of organisms is found which 

 partake equally of animal and vegetable charac- 

 ters, or, rather, exhibit properties which are com- 

 mon to them both, we shall hesitate to postulate 

 confidently for the primitive antecedence of vege- 

 table life, although, perhaps, in later epochs the 

 preexistence of vegetables may be looked upon 

 as necessary to the life of more developed animal 

 organisms. 



The reflection forces itself upon us that we 

 are just as ignorant of the mode of first origin of 

 all the compounds of the inorganic elements as 

 we are of that of living matter ; and we may 

 therefore be excused if we suspend all theory and 

 conjecture until we shall be guided to more reli- 

 able hypotheses through the plain track of obser- 

 vation and experiment. 



The practical applications of the increased 

 knowledge of the origin of minute animal and 

 vegetable organisms are so numerous that it would 

 occupy a much longer time than is at my dispos- 

 al to give any detailed account of them ; but they 

 are of such immense importance in their com- 

 mercial, social, and sanitary relations, that they 

 ought never to be lost sight of. 



It is now proved beyond doubt that the origin 

 of putrefaction and fermentation is dependent on 

 the presence in the substances which are the seat 

 of change in these processes, or in the surround- 

 ing air, of the germs of minute organisms of an 

 animal or vegetable nature, and that the mainte- 

 nance of the chemical changes in which these 

 processes mainly consist is coincident with and 

 casually (if not essentially) dependent upon the 

 growth and multiplication of these organisms. 



Prof. Lister had the merit of being the first 

 to apply the germ-theory of putrefaction to ex- 

 plain the formation of putrid matters in the living 

 body ; and he has founded on this theory the now 

 well-known antiseptic treatment of wounds, the 

 importance of which it would be difficult to over- 

 estimate. 



The success or failure of plans for the pres- 

 ervation of meat and other articles of food with- 

 out question depends on the possibility of the 

 complete exclusion of the germs which are the 

 cause of putrefaction and fermentation ; and their 

 management must therefore be founded on the 

 most accurate knowledge of these organisms, and 



