IV. AETIOLOGY
Aetiology: Causes of the Phenomena of Life. Origin of Living Matter - Abiogenesis and Biogenesis.
Morphology, Distribution, and Physiology investigate and determine the fact of Biology. Aetiology has for its object the ascertainment of the causes of these facts, and the explanation of biological phenomena, buy showing that they constitute particular cases of general physical laws. It is hardly needful to say that aetiology, as thus conceived, is in its infancy, and that the seething controversies, to which the attempt to found this branch of science made in the Origin of Species has given rise, cannot be dealt with in the limits of this article. At most, the general nature of the problems to be evolved, and the course of inquiry needful for their solution, may be indicated.
In any investigation into the causes of the phenomena of life, the first question which arises is, whether we have any knowledge, and if so, what knowledge, of the origin of living matter?
In the case of all conspicuous and easily – studied organisms, it has been obvious, since the study of nature began, that living beings arise by generation from living beings of a like kind; but before the latter part of the 17th century, learned and unlearned alike shared the conviction that this rule was not of universal application, and that multitudes of the smaller and more obscure organisms were produced by the fermentation of non – living, and especially of putrefying dead matter, by what was then termed generatio aequivoca or spontanea, and is now called abiogenesis. Redi showed that the general belief was erroneous in a multitude of instances; Spallanzani added largely to the list; while the investigations of the scientific helminthologists of the present century have eliminated a further category of cases in which it was possible to doubt the applicability of the rule "omne vivum e vivo" to the more complex organisms which constitute the present fauna and flora of the earth. Even the most extravagant supporters of abiogenesis at the present day do not pretend that organisms of higher rank than the lowest Fungi and Protozoa are produced otherwise than by generation from pre-existing organisms. But it is pretended that Bacteria, Torulae, certain Fungi, and "Monads’ are developed under conditions which render it impossible that these organisms should have proceeded directly from living matter.
The experimental evidence adduced in favour of this proposition is always of one kind, and the reasoning on which the conclusion that abiogenesis occurs is based may be stated in the following form:--
All living matter is killed by being heated to n degrees.
The contents of the closed vessel A have been heated to n degrees.
Therefore, all living matter which may have existed therein has been killed.
But living Bacteria, &c., have appeared in these contents subsequently to their being heated.
Therefore, they have been formed abiogenetically.
No objection can be taken to the logical form of this reasoning, but it is obvious that its applicability to any particular case depends entirely upon the validity, in that case, of the first and second propositions.
Suppose a fluid to be full of Bacteria in active motion, what evidence have we that they are killed when the fluid is heated to n degrees? There is but one kind of conclusive evidence, namely, that from that time forth no living Bacteria make their appearance in the liquid, supposing it to be properly protected from the intrusion of fresh Bacteria. The only other evidence, that, for example, which may be furnished by the cessation of the motion of the Bacteria, and such slight changes as our microscopes permit us to observe in their optical characters, is simply presumptive evidence of death, and no more conclusive than the stillness and paleness of a man in a swoon are proof that he is dead. And the caution is the more necessary in the case of Bacteria, since many of them naturally pass a considerable part of their existence in a condition in which they show no marks of life whatever save growth and multiplication.
If indeed it could be proved that, in cases which are not open to doubt, living matter is always and invariably killed at precisely the same temperature, there might be some ground for the assumption, that, in those which are obscure, death must take place under the same circumstances. But what are the facts? It has been pointed out at the commencement of this article, that the range of high temperatures between the lowest, at which some living things are certainly killed, and the highest, at which others certainly live, is rather more than 100° Fahr., that is to say, between 104° Fahr. and 208° Fahr. it makes no sort of difference to the argument how living beings have come to be able to bear such a temperature as the last mentioned; the fact that they do so is sufficient to prove that, under certain conditions, such a temperature is not sufficient to destroy life.
Thus it appears that there is no ground for the assumption that all living matter is killed at some given temperature between 104° and 208° Fahr.
But, further, there is very strong reason for believing that the influence of temperature on life is greatly modified, first, by the nature of the medium in which organisms are placed, and, secondly by the length of time during which any given temperature is kept up.
On this point recent experiments made by Dr. Roberts of Manchester are of great importance. He found, for example, as every other careful experimenter has done, that ordinary infusion of hay boiled for a few minutes was sterilized, that is to say, no development of Bacteria took place in it, however long it might be kept; while if the infusion was rendered alkaline with ammonia or liquor potassae, it was not sterilized except after an exposure to the heat of boiling water for more that an hour. Sometimes it became productive after two hours, and once after three hours of such exposure. Is it to be imagined that, in the case of the alkalized hay infusion, the heat applied really killed the Bacteria which existed in the infusion, and that Bacteria of identically the same kind were generated afresh out of the dead matter? Or is it more probable that the powers of resistance of the Bacteria to heat were simply increased by the alkalinity of the infusion? The statement of the questions surely render it unnecessary to answer them.
Dr. Roberts further proves there are two factors in the indication of sterilization, the degree of heat on the one hand, and the duration of its application on the other. A longer exposure to a lower temperature was equivalent to a shorter exposure to a higher temperature. "For example, speaking roughly, an exposure of an hour and a half to a heat of 212° Fahr. appeared to be equivalent to an exposure for fifteen minutes to a heat of 228° Fahr." (FOOTNOTE 1)
It is hard to conceive what explanation can be offered of this fact, except that, under the conditions of the experiment, the organisms were either all affected by the first incidence of the heat in such a way as only to arrest some of their vital functions, and to leave a potentially of life in them, such as exists in some kinds of dried living matter; or that they individually differed very much in their powers of resistance, and that some were able to withstand heat much longer than others.
Under these circumstances it will be evident, that no experimental evidence that a liquid may be heated to n degrees, and yet subsequently give rise to living organisms, is of the smallest value as proof that abiogenesis has taken place, and for two reasons:-- Firstly, there is no proof that organisms of the kind are dead, except their permanent incapacity to grow and reproduce their kind; and secondly, since we know that conditions may largely modify the power 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 afresh out of dead matter.
Not only is the kind of evidence adduced in favour of abiogenesis logically insufficient to furnish proof of its occurrence, but it may be stated as a well-based induction, that the more careful the investigator, and the more complete his mastery over the endless practical difficulties which surround experimentation on this subject, the more certain are his experiments to give a negative result; while positive results are no less sure to crown the efforts of the clumsy and the careless.
FOOTNOTE
1. Proceedings of the Royal Society, No. 152, p. 290.
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