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As regards the second problem offered to us by Redi, whether Xenogenesis obtains, side by side with Homogenesis, whether, that is, there exist not only the ordinary living things, giving rise to offspring which run through the same cycle as themselves, but also others, producing offspring which are of a totally different character from themselves, the researches of two centuries have led to a different result.
The spores could not be made to give rise to such germs by cultivation; nor were such germs discoverable in the air, or in the food of the fly. It looked exceedingly like a case of Abiogenesis, or, at any rate, of Xenogenesis; and it is only quite recently that the real course of events has been made out.
The term Heterogenesis, however, has unfortunately been used in a different sense, and M. Milne-Edwards has therefore substituted for it Xenogenesis, which means the generation of something foreign. After discussing Redi's hypothesis of universal Biogenesis, then, I shall go on to ask how far the growth of science justifies his other hypothesis of Xenogenesis.
It is obvious that this depends upon the way in which the Panhistophyton is generated. If it may be generated by Abiogenesis, or by Xenogenesis, within the silkworm or its moth, the extirpation of the disease must depend upon the prevention of the occurrence of the conditions under which this generation takes place.
But if we turn to pathology, it offers us some remarkable approximations to true Xenogenesis. As I have already mentioned, it has been known since the time of Vallisnieri and of Réaumur, that galls in plants, and tumours in cattle, are caused by insects, which lay their eggs in those parts of the animal or vegetable frame of which these morbid structures are outgrowths.
The 'Physical basis of life' has been driven into a corner, hunted down, seized at last, and over the heads of an eager, panting, chasing generation, is triumphantly dangled this 'Scientific Fox' brush, 'Nucleated Protoplasm, the structural unit! But how or whence sprang the laws of 'Protein'? Hatred of certain phrases is more bitter than of the principles they express, and because theologians cling to the words God, Creative Acts, Divine Wisdom, Providential Adaptation, scientists declare them the dicta of ignorance, superstition, and tradition, and demand that we shall bow before their superior wisdom, and substitute such terms as 'Biogenesis, 'Abiogenesis, and 'Xenogenesis. But where is the economy of credulity?
The beef is dead ox, and the hay is dead grass; but the "organic molecules" of the beef or the hay are not dead, but are ready to manifest their vitality as soon as the bovine or herbaceous shrouds in which they are imprisoned are rent by the macerating action of water. The hypothesis therefore must be classified under Xenogenesis, rather than under Abiogenesis.
Now arises the question, are these microzymes the results of Homogenesis, or of Xenogenesis? are they capable, like the Toruloe of yeast, of arising only by the development of pre-existing germs? or may they be, like the constituents of a nut-gall, the results of a modification and individualisation of the tissues of the body in which they are found, resulting from the operation of certain conditions?
The tapeworms, bladderworms, and flukes continued to be a stronghold of the advocates of Xenogenesis for a much longer period.
If there were a kind of diseased structure, the histological elements of which were capable of maintaining a separate and independent existence out of the body, it seems to me that the shadowy boundary between morbid growth and Xenogenesis would be effaced. And I am inclined to think that the progress of discovery has almost brought us to this point already. I have been favoured by Mr.
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