The story which has just been told of the function which is expressed in the term "generation" may be considered from more than one point of view.
In the record of the successive changes which end in the production of the young mammal resembling its parents in form, constitution, temperament, and susceptibilities, the scientist sees an example of the process of evolution, so familiar that it fails to excite any special notice. The less experienced observer, with a larger perception of the marvellous, cannot avoid being impressed with the remarkable results of apparently simple causes. Opponents of the theory of evolution have found some amusement in quoting with derision the statement which someone is supposed to have made, that man arose from the jelly-fish by a series of developmental changes occupying ages. Most probably no one has been asked to believe in such an origin of the human race; but it may be worth while to think for a moment on the facts which have just been recorded, about which there is no dispute, all tending to prove that an organism much less advanced in the scale of creation than a jelly-fish, being, indeed, only a speck of germinal matter, is capable of evolving a man.
The mammalian ovum is in reality a minute speck of animal matter having no individuality, a simple cell formed by investing membrane surrounding an albuminous mass, having a diameter of less than the 1/1000th of an inch, containing a germinal vesicle and a germinal spot only visible under the highest powers of the microscope. It is but required that the minute germ in the egg, or ovulum as it may more appropriately be called, should be fertilized by contact with the male sperm to ensure the development of a man, or a much larger mammal, not during the course of ages, but in a few months. If it could be demonstrated that the higher mammalian is the outcome of inconceivably prolonged transformations in the organism of the jelly-fish, it is difficult to understand that there would be any greater ground for wonder than should naturally be, at the contemplation of the metamorphosis of the mammalian ovum, ending in the evolution of the highest animal in creation.
Multiplication of the species in the minute, lowly-organized beings which may be described as constituting the dawn of life, is effected by processes which may be termed marvellous in their extreme simplicity.
Taking the amoeba for an example, we have a mere film of transparent germinal, i.e. living, matter, capable of movement without any discernible organs of locomotion, breathing without any respiratory apparatus, and taking necessary nutriment and growing thereby without a trace of digestive organs. Multiplication of these primitive forms of living things is the simple result of the separation of portions of the mass, which are at once new beings possessing all the powers and properties of their parent. Among the Infusoria, the highest division of the Protozoa, there are endless varieties of form, most of them moving freely in the fluid in which they live, by the aid of fine hair-like projections (cilia), although some of them have a stationary life, being attached to stones or other bodies. Their reproductive powers are always active, and result in the growth of buds, which project from their bodies, become severed from their parents, and enjoy an independent existence, giving origin to new lives by the simple process of budding in their turn. Division of the organisms is another method of multiplication which is common. The paramecium, for instance, has been seen to divide into several parts, which go on dividing every twenty-four hours. Monads, which are the smallest of infusorial animalcules, exhibit phenomena closely allied to those which have been described in the early changes in the mammalian ovum, i.e. multiplication by cleavage. A small fissure is observed in the cell wall at two, sometimes at four, points, and by the simple extension of the fissures the creature is converted into two or four individuals.
Fig. 543. - Examples of Multiplication by Division and by Budding.
1. Amoeba; p, point of separation. 2. Cbilodon cucullulus; successive stages of division. 3. Hydra fusea; a, very young buds; b, older buds at different stages.