Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/25

Rh The simplest forms of life known are the Monera (Figs. 1, 2, 3), which may be defined as living jelly,—formless, structureless, in every sense of the word. Their movements are restricted to a gliding or crawling, a drawing in or putting out of their jelly-like body; their reproduction is a simple splitting of their body into two halves, each half becoming a new Monas. Such a living slime is seen in Protogenes. The first sign of structure we meet with in this kind of being is where a wall has been exuded inclosing the jelly-like body, as in Protomonas, or as in Amoeba (Figs. 4, 5, 6, 7), where the slime has aggregated in the middle, forming a nucleus. These two different conditions, a nucleated slime and a walled slime, are combined in Arcella, these last being undistinguishable from the young of the simplest water-plants (Algae) and the undeveloped forms of certain jelly-fish (Siphonophorae). In the springtime the ponds are often covered with a green matter, which, when examined with a microscope, is found to consist of Euglena (Fig. 8), minute flask-shaped bodies with little tails; when these bodies are covered with hairs (cilia) they are known as Peridnium. They, like the Arcella, cannot be distinguished from the young of the simplest plants and animalcula (Infusoria). What conclusions can be drawn from the existence of Monera, Amoebae, Euglena? How have they originated? Either they have come from pre-existing forms, or have arisen through spontaneous generation. Where have the pre-existing forms come from, is immediately suggested by the first answer, which only waives the question, and is therefore no answer at all. It will not suffice to say that they were created; as well might an astronomer explain the motion of the moon around the earth by saying it was created so to move. What is meant by spontaneous generation?