12 oo PHYSIOLOGY 



would, by growth and division, in the course of a year form a mass of proto- 

 plasm the size of the earth, assuming of course that no accidents or destruc- 

 tive agencies intervened to destroy the paramcecia which were being formed. 

 This computation, which may seem a fanciful one, is useful as indicating the 

 enormous number of individuals brought under the action of natural selec- 

 tion, which very few survive. In unicellular organisms, such as para- 

 mcecium 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 themselves 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 invironment. Any such 

 modification 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. 



Reproduction by continuous growth and division is not, however, the 

 only means, even in the unicellular animals, by which new generations 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. ' conjuga- 

 tion,' 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 

 exchange of micro-nuclei, one micro-nucleus from A passing into B, while 

 one micro-nucleus from B passes into A. The two cells then separate, a single 

 micro- nucleus being formed in each by the amalgamation of the two. This 

 micro-nucleus then divides three times, so that eight nuclei are formed, while 

 the cell itself divides into four, two nuclei passing into each of the daughter 

 cells. Of these one enlarges to form the macro-nucleus, while the other 

 remains as the micro-nucleus. After conjugation has occurred, the colony of 

 infusoria takes on, so to speak, a new lease of life, and there is a rapid 

 production of new generations by simple division of the cells, in which both 

 macro-nucleus and micro-nucleus take part. Conjugation apparently only 



