1342 PHYSIOLOGY 



dissimilation, of growth and activity, is only possible so long as cell 

 division keeps pace with growth. In unicellular organisms, under 

 favourable conditions, this growth and multiplication occur with 

 prodigious rapidity. It has been computed that a paramcecium, 

 freely supplied with food material, would, by growth and division, in 

 the course of a year form a mass of protoplasm the size of the earth, 

 assuming of course that no accidents or destructive agencies intervened 

 to destroy the paramcecia which were being formed. This computa- 

 tion, which may seem a fanciful one, is useful as indicating the enor- 

 mous number of individuals brought under the action of natural 

 selection, which very few survive. In unicellular organisms, such 

 as paramoscium or amoeba, death cannot be regarded as a natural 

 process. They may be eaten by higher organisms or serve as food to 

 vegetable parasites, but so long as conditions are favourable and food- 

 supply sufficient, they will continue to grow and reproduce them- 

 selves eternally. In the course of its existence each individual may be 

 brought under many varieties of conditions ; some of these may be 

 so harmful that the individual is destroyed and its race comes to an 

 end. Other individuals, under circumstances of less severity, may 

 undergo modifications in their molecular structure which will serve to 

 neutralise the effect of the injurious environment. Any such modifica- 

 tion in structure, morphological or molecular, must be transmitted to 

 the next generation, so that with slowly varying external conditions 

 there is a possibility of a corresponding slow variation in type, which 

 may finally attain a form altogether different from that with which 

 it set out. A new species may in this way be formed by gradual 

 alteration of environment. It is not therefore difficult to understand 

 in the case of such organisms either the maintenance of type by heredity 

 under constant conditions, or the change of type with gradually 

 varying conditions. 



Keproduction by continuous growth and division is not, however, 

 the only means, even in the unicellular animals, by which new genera- 

 tions may be produced. If protozoa, such as paramcecia, be kept for 

 a long time in nutrient solutions their rapidity of reproduction after a 

 time falls off, while many die, and others become the easy prey to 

 infectious diseases. Under these conditions a new phenomenon makes 

 its appearance, viz. ' conjugation,' which is the analogue of the sexual 

 reproduction of the higher animals. Infusoria contain two kinds of 

 nuclei, a large and a small, known as the macro-nucleus and the micro- 

 nucleus respectively. During conjugation the macro-nucleus breaks 

 up and disappears in two cells, which become closely applied together, 

 while in each the micro-nucleus divides twice to form four spindle- 

 shaped bodies. Three of these degenerate (like the polar bodies of 

 the ovum), while the fourth divides into two. This is followed by an 



