Before we can be at liberty to conclude that something is universally true because we have never known an instance to the contrary, we must have reason to believe that if there were in nature any instances to the contrary, we should have known of them." Now it was only by an utter inattention to this latter allimportant requirement that the "past experience of mankind" could ever have appeared to warrant the induction omne vivum ex vivo.

As Mr. Mill pointed out,* the proposition, "all swans are white," must have appeared to Europeans, not many years ago, an "unequivocal instance of uniformity in the course of nature." Subsequent experience has shown that they were mistaken, although they and all their predecessors through many centuries had observed nothing to contradict this proposition. "The uniform experience therefore of the inhabitants of the known world, agreeing in a common result, without one known instance of deviation from that result, is not always sufficient to establish a general conclusion."

(b). The exposure of the untruth of certain old and crude doctrines concerning 'spontaneous generation,' many of which date from the earliest times; and the fact that the belief in this mode of generation has been successively driven, with increasing knowledge, Loc. cit. vol. i., pp. 348, 351.

*

from higher to lower forms of life, till at the last it is maintained as a mode of origin only for the very lowest and most minute of living things, has been regarded by many (as I have already pointed out) as one of the most weighty arguments against this kind of generation. But this objection, as before shown, is robbed of all its seeming strength when it is said that the modern Evolutionist would only expect to obtain evidence concerning the de novo origin of the minutest specks of Living Matter-gradually emerging into the region of the visible and subsequently developing into the most elementary Forms of Life.

Thus the formula, omne vivum ex vivo, has even no sufficient à priori warrant. It is an induction which has been formed after a partial scrutiny of the facts-one that has been arrived at, not in accordance with the modern methods of experimental inquiry, but by the ancient custom of mere passive observation and enumeration, against which the founder of the Inductive Philosophy so strongly raised his voice.*

So far, then, it would seem that at least as much is to be said in favour of the new as of the old hypothesis, even from a mere prima facie considera

* Named by him "Inductio 'per enumerationem simplicem, ubi non reperitur instantia contradictoria."

tion of their merits, whilst a closer scrutiny of their respective foundations tells much more in favour of the new hypothesis of Archebiosis; in the first place, because no reason can be shown why the process of life-evolution should have been arrested; and secondly, because if it does occur at the present time, it never could come under the direct observation of anybody, and consequently the general experience of mankind concerning the ' reproduction' of living things, upon which the second hypothesis and the dictum omne vivum ex vivo have been founded, would in no way be questioned the facts would lie altogether outside this experience.*

But a belief in Archebiosis, whether past or present, seems to me necessarily to carry with it a belief in Heterogenesis. So that if Archebiosis be continually taking place, Heterogenesis should be an equally common phenomenon. And even for those who believe that Archebiosis took place in the past though it has now ceased, Heterogenesis would remain as a very possible and even probable process from an

* As I have elsewhere said :- "Living matter, like crystalline matter, is only formable by a synthesis of its elements. As crystals have not the power of self-multiplication, they have only one mode of origin. But because organisms have reproductive powers, the obviousness of these modes of increase has sufficed to cast doubts upon the reality of the independent origin of living units."-The Beginnings of Life, vol. ii., p. 77.

à priori point of view. Such a process is, in fact, for living matter what Allotropism is for crystalline matter. And it so happens that the evidence in favour of the present occurrence of Heterogenesis is even stronger and much more varied than that in favour of the present occurrence of Archebiosis. Whilst the latter is a strongly warranted inference, the former is a matter of direct observation. So that concerning the present occurrence of Heterogenesis we may say, (1), that it follows as an almost necessary consequence from the physical doctrines of life; (2), that it is a process which admits of daily observation by skilled observers; and (3), that it explains many series of phenomena of the most varied nature, which would otherwise remain quite inexplicable.*

* It is worthy of note, moreover, that it is the recognition of the present occurrence of Heterogenesis which is the il-important necessity. A belief of this kind will carry with it all those important changes in biological doctrine and in medical science which seem to me both necessary and inevitable. The further belief as to the present occurrence of Archebiosis, is an extension of the 'Spontaneous Generation' doctrine, which, though it may be logically demanded and warranted, is one of altogether secondary importance in relation to the changes of doctrine that it would involve. This state of things is the more important, because a belief in Heterogenesis is open to biologists of all shades of opinion. Indeed those biologists who still believe in the existence of a special vital principle,' would in all probability only ifner the occurrence of Heterogenesis from such experimental facts as would warrant, on the part of the Evolutionist, a belief in the present occurrence of Archebiosis. The Evolutionist, however, is bound to recognize a difference between living organic matter and dead organic matter, which the Vitalist, holding himself aloof from positive scientific

Our present position may perhaps be best illustrated by tabulating in parallel columns a statement of the principal reasons and facts which seem to support the hypothesis of the present occurrence of Archebiosis and Heterogenesis, but which are more or less inexplicable by the hypothesis of Panspermism and an exclusive Life-transmission doctrine. I am compelled to arrange the matter in this apparently one-sided form, because I know of no large classes of facts adverse to the hypothesis of the present continuance of Archebiosis and Heterogenesis.

In support of the present occurrence of Archebiosis.

1. Our belief in the Continuity of natural phenomena seems to require it. -по

2. The fact that crystalline matter still comes into being, or originates under the majority of those conditions in which its growth occurs.

3. The fact that the microscopical evidence in favour of origination is similar in the case of crystalline and living matter— both appear to arise de novo.

evidence so far as these particular doctrines are concerned, might not feel called upon to admit.-See "The Beginnings of Life," vol. i., Pp. 244-249.