Professor Lankester's paper is full of entertaining facts of his own discovery, but a number of experiments made by Mr. Blomfield, of Oxford, and University College, London, and quoted by Professor Lankester, are of special interest to us in this connection, as they furnish some evidence that the green Hydra does, like Convoluta, evolve oxygen in the sunlight. The observations are incomplete, but nevertheless of much value as they go to establish a second case in which animals destitute of yellow cells and pigment-bodies, but endowed presumably with true chlorophyl, do actually give off oxygen.
Meantime the reader can not have failed to perceive that the question as to the evolution of oxygen has become of secondary importance. It is nothing strange if algæ living in animals give off oxygen by virtue of their chlorophyl. In any special case we must now first inquire—Are the colored parts mere plants dwelling within the animal, or are they not? If not, then we must, if possible, apply the spectroscope for the study of the pigment (the chlorophyl group giving rather characteristic spectra), and then, if chlorophyl is present, test, if we can, for oxygen elimination. It is tolerably clear that the occurrence of native chlorophyl in animal protoplasm is not so wide-spread as was suspected before symbiosis was detected; yet the cases of Hydra, Spongilla, and Convoluta are still unsettled, and others may be added to their number: it must be granted, however, that the indications seem to be that in some cases animals may possess veritable chlorophyl arranged as in plants, giving the same spectrum and having the same power over carbonic acid.
At present it will be far more profitable to consider the significance of symbiosis than to speculate upon the result of observations belonging to the future. Professor Semper, in his work entitled "Animal Life," reminds us that, if it should come to pass [as it has] that we must consider much of the chlorophyl found in animals to be borne by vegetable messmates, we need not be surprised; since lichens—formerly supposed to be simple vegetables—have now been shown to be associated organisms—a fungus parasitic upon algæ. There is, indeed, much superficial resemblance between the two phenomena, and it is said to have been from the literature of lichens that the expressive word, symbiosis, was borrowed. In truth, there is really less analogy than at first appears; and, as there is no reason for considering the lichens as other than interesting and complicated cases of parasitism, we may hereafter, I think, reserve the word symbiosis for the description of that very different association of algæ with animals which it has been the purpose of the writer to elucidate. The word zoöphyte might, indeed, be used here with an accurate meaning had it not already a very definite (though utterly senseless) use in pseudoscientific books and minds.
Mr. Geddes pictures at considerable length the probable physiological relationship between the organisms associated in symbiosis;