In the case of _Paramœcium_ the process of multiplication is slightly more complex than that of _Amœba_ in the fact that sometimes before the simple fission of the body takes place the interesting phenomenon of conjugation occurs. _Paramœcium_ may reproduce itself for many generations by simple fission, but a generation finally appears in which conjugation takes place. Two individuals come together and each exchanges with the other a part of its nucleus. Then the two individuals separate and each divides into two. The result of the conjugation, or the coming together, of two individuals with mutual interchange of nuclear substance is to give to the new _Paramœcia_ produced by the conjugating individuals a body which contains part of the body-substance of two distinct individuals. If the two conjugating individuals differ at all--and they always do differ, because no two individual animals, although belonging to the same species, are exactly alike--the new individual, made up of parts of each of them, will differ slightly from both. Nature seems intent on making every new individual differ slightly from the individual which precedes it. And the method of multiplication which Nature has adopted to produce the result is the method which we have seen exhibited in its simplest form in the case of _Paramœcium_--the method of having two individuals take part in the production of a new one. The development of the new _Paramœcia_ is a little more complex than that of _Amœba_. Not only must the new _Paramœcium_ grow to the size of the original one, but it must develop those slight, but apparent, modifications of the parts of its body which we can recognize in the full-grown, fully developed _Paramœcium_ individual. A new mouth-opening must develop on the new individual formed of the hinder half of the original _Paramœcium_ and new cilia must be developed. Thus there is a slight advance in complexity of development, just as there is in complexity of structure in _Paramœcium_ as compared with _Amœba_. In the many-celled animals this complexity of development is carried to an extreme. =Birth and hatching.=--When a young animal is born alive, it usually resembles in appearance and structure the parent, although of course it is much smaller, and requires always a certain time to complete its development and become mature. A young kangaroo or opossum is carried for some time after its birth in an external pouch on the mother's body and is a very helpless animal. A young kitten is born with eyes not yet opened and must be fed by the mother for several weeks. On the other hand young Rocky Mountain sheep are able to run about swiftly within a few hours after birth.