IV. 'ETIOLOGY has for its object the ascertaining of the causes of the facts developed under the preceding heads of morphology, distribution, and physiology, by showing that they constitute partibular cases of general physical laws. It is well to say that is vet ill its infancy, and that no extended dissertation touching the origin of the species is here undertaken. We can only indicate the general nature of the problems to be involved, and the cour3e of inquiry that may lead to their solution. The first is: Have we any knowledge, and, if so, what knowledge, of the origin of living matter? Down almost to our times, the universal assumption was that beings were produced by generation from previous living beings of the same kind; but about 200 years ago investigators began to suspect that this rule was not universally applicable, but that small and obscure organisms were produced by the fermentation of dead putre fying, non-living matter, by a process which i hey called spontanea or generatio aquicoca, now known as abiogenesi.i. After the investigations of Redi, Spallanzi, and others, people began to doubt the applicability of the axiom "all life comes from the living" to the more complex organisms which constitute tlic present fauna and flora. The most ardent supporters of abiogenesis at the present time do not pretend that organ isins of higher rank than the lowest fungi and protozoa arc produced otherwise than by generation from pre-existing organisms. It is. however. alleged that bacteria, typifies, and certain fungi, and mounds are developed under conditions which render it impossi ble that these organisms should have proceeded directly front living matter. The experi mental evidence adduced in favor of this proposition is always of one kind, and the reasoning on which the conclusion that abiogenesis occurs is based may be thus stated: 1. All living matter is killed when heated to certain degrees. 2. The contents of a cer tain closed vessel have been heated to such degrees. 3. Therefore, all living matter which may have been therein has been killed. But, living bacteria have appeared in such contents subsequently to their being heated; therefore, they have been formed abiogenetically—that is, a living being has collie from non-living matter. This is per fect logic; but then its validity depends upon the absolute accuracy of the first and second propositions. Suppose we have a fluid full of active bacteria; what evidence
have we that they are killed by the heat? Only one kind of evidence can be conclusive, and that is that the fluid has been carefully protected from outward contact, and that bacteria have never appeared since the heating. The other testimony, for example that which may be furnished by the cessation of motion in the bacteria, and such changes as microscopes allow us to observe, is merely presumptive evidence of death, but no more conclusive of death than are insensibility and paleness in a man who has swooned. But though some living beings are killed with moderate heat, and some bear a very high degree without dying, there is no ground for the assumption that aq living matter is killed at some given temperature. There is, further, good reason for believing that the influence of temperature on life is greatly modified; first, by the nature of the medium in which the organisms to be placed, and, secondly, by the length of time dar ing which they are subjected to trial. The latest experiments leave the question as far as ever from settlement; hence it is reasoned that no experimental evidence that a liquor may be heated to certain degrees and yet snbscqucntly give rise to living organisms, is of the least value as proof that abiogenesis has taken place; and this for two reasons: 1. There is no proof that organisms of the kind in question are dead, except their inca pacity to grow and reproduce their kind. 2. Since we know that conditions may largely modify the pcmer of resistance of such organisms to heat, it is far more probable that such conditions existed in the experiment in question than that the organisms were generated anew out of dead matter. Prof. Huxley considers not only that the kind of in favor of abiogenesis is logically insufficient to furnish proof of its occurrence, but also that it may be stated as a wel•based induction that the more careful the inves tigator, and the more complete his mastery over the endless practical difficulties which surrouud experimentations on this subject, the more certain arc his experiments. to give a negative result, while the positive results are no less sure to crown the efforts of the clumsy and the careless.