The above may be deemed one of the most interesting zoological theories that has ever been promulgated in modern times. It was founded by Chamisso, and termed the 'alternation of generation,' but was much improved and extended by the researches of Steenstrup. Professor Owen, however, had previously reduced the theory to a fixed and definite scientific form, under the title of '_Parthenogenesis_.' Another author, not viewing the Medusæ in the various stages of development as an aggregation of individuals, 'in the same sense that one of the higher animals is an individual,' proposes that each Medusa be considered as an individual, developed into so many 'zooids.' Into this abstract question, of course, I cannot enter. The reader who would wish to know more of the subject than I have faintly shadowed forth in this chapter, may consult Steenstrup's Memoir, published by the Ray Society; Dr. Reid's admirable papers in the 'Magazine of Natural History' 2d series; Lewes' 'Sea-Side Studies;' and the learned works of Professor Owen. [Illustration: MEDUSÆ IN VARIOUS STAGES OF DEVELOPMENT.] CHAPTER XIV. Doris, Eolis, &c. 'The inhabitants of the watery element were made for wise men to contemplate and fools to pass by without consideration.'--ISAAK WALTON. XIV. One fine morning during the month of January, on peering into my largest tank I perceived, attached to the upper portion of the marble arch (centre piece), a peculiar object that had evidently been deposited during the preceding night, but by whom or by what means I knew not. It resembled a fungoid growth, or riband of flesh, plaited up and attached at one edge to the stone. At every undulation of the water the object moved to and fro with an extremely graceful motion. By careful and close examination it was seen to be covered with a film, that gradually expanded until it burst, and thus gave means of escape to thousands of minute white granules. On submitting these to the microscope, a most wondrous sight met my astonished eyes. Each dot or granule proved to be a transparent shell, resembling the periwinkle or rather the nautilus in shape; containing an animal whose excited and rapid movements were amusing to witness. From out the opening of the shell appeared now and again two rings of cilia. When these organs were about to be put into action, they reminded me of two circular tubes of gas connected together, and each containing innumerable perforations, which were sometimes suddenly and entirely lit up by a torch being applied to one end. The _cilia_ may be distinctly seen to play at a certain point, and then gradually extend round the circumference of the rings. When the whole are in full action their movements are so extremely swift _as to appear devoid of motion_, and thus bear a resemblance to rings of flame.