Since the oldest times attempts have been made to find some features characteristic respectively of animals and of plants. But all of these criterions, one by one, have had to be abandoned. Finally, Haeckel, of Jena, severed the Gordian knot. He created an intermediate realm to which all is consigned that is not distinctively a plant or an animal. Now, however, the difficulty is only greater than it was before. Formerly the question was. Animal or vegetable kingdom? Now it has become, Animal, protista (for thus Haeckel named this new division), or vegetable kingdom?
This system of classification at least affords us a general view over those organisms which are, as it were, the connecting links between the vegetable and the animal world, between which, at first sight, there seems to be an impassable chasm.
The slime-fungi have before been alluded to; the class of fungi embraces yet other groups that, as animal-plants, call for some notice on our part. Above all must be named the bacteria; in 1853 these organisms were relegated to the vegetable kingdom. In the case of many bacteria, motion can be observed; some move quietly about, others slide and glide to and fro like snakes or eels. A few species are provided with special thread-like filaments for this purpose. Some bacteria, as well as many other kinds of organisms, can withstand—that is to say, survive—considerable heat. There are, for instance, the algæ in the waters of the Carlsbad Sprudel; they attain luxuriant growth at a temperature of 54°C. Other species of this kind live though exposed to the hot vapors (about 65°C.) of Ischia and Liparia. This is all the more remarkable as protoplasm, the albuminoid substance on which the phenomena of life depend, curdles at a much lower temperature.
A similar tenacity of life has been observed also in plants of a higher order. Wheat-kernels, for instance, if they have been previously well dried, lose the property of germinating only at 72°C, barley at 65°C.; if moist, however, they die at a temperature of about 50°C. The seeds of many leguminous plants can not survive a temperature of 35°C. Pouchet, however, has observed that seeds which were found in the unwashed wool of Brazil sheep resisted for four hours the action of boiling water. The hard seed-shells had prevented the entering of the water. Haberlandt in 1863 made experiments with the seeds of eighty-eight different kinds of cultivated plants, and found that some of them could, when in a perfectly dry condition, stand, for forty-eight hours, exposure to a temperature of 100°C.; in some instances, in fact, germination was hastened by the higher temperature. Nor is the power of germination always destroyed by intense cold. This is demonstrated by the fact that, of three hundred wheat-kernels which were left north by the Polaris in 1871, some sixty germinated in 1877.
Let us now turn our attention to a different group of organisms,
