686 The Outline of Science 



of growth and organisation; and in animals with more compli- 

 cated types of budding, whole chains of buds are formed, and 

 new individuals are thus produced in rapid succession. 



Many disadvantages of fission are obviated by budding; but 

 it would scarcely work with animals which possess a complicated 

 skeleton; and besides, the ordinary body tissues of the highest 

 animals have lost the power of unlimited growth, needful if buds 

 are to be formed. 



However, as soon as multicellular animals and plants had 

 been evolved, the sexual process inevitably became associated 

 with reproduction. The sexual process implies the union of two 

 single cells into one, and thus to effect it two cells must be 

 detached from the multicellular animals to which they belong, 

 and the cell produced by their union must multiply and grow into 

 a new many-celled individual. 



A sexual process, however, does occur in many unicellular 

 animals. In such as the Slipper Animalcule, for instance, the 

 asexual reproduction by fission will take place once or twice a 

 day, and may continue for a great number of generations through 

 weeks or months or even years. At intervals, however, this cycle 

 or unlimited multiplication is broken by what is called conjuga- 

 tion. Individuals come together, mouth to mouth, in pairs ; their 

 fluid internal substance comes in contact through their mouths; 

 a complicated division of their nuclei (which, as we have seen in 

 a previous chapter, are the bearers of hereditary qualities) occurs; 

 and finally one nucleus from either member of the pair travels 

 across and unites with a stationary nucleus in the other member. 

 Thereafter, the two separate, and embark upon a new sexual 

 cycle of fission. 



A similar process occurs in most, perhaps in all, unicellular 

 animals and plants: only in the Bacteria, which can hardly be 

 styled cells, does it seem to be universally absent. Often the 

 process is less complicated than in the Slipper Animalcule; two 

 individuals simply come together and unite, first their bodies and 



