l] REPRODUCTION 7 



round its cage it is obeying the stimulus of. hunger. In the first 

 case we have to deal with an external stimulus, in the second with 

 an internal one. Of course since all internal changes are ultimately 

 due to changes in the surrounding medium, e.g. hunger to a dis- 

 appearance by digestion of the food in contact with the stomach, 

 the distinction between external and internal stimuli, though con- 

 venient, cannot be sharply drawn. The power of protoplasm to 

 originate movement in consequence of internal changes is called 

 automatism. In the case of external stimuli we can often observe 

 that the disturbance caused at the point of application of thq 

 stimulus is propagated to widely different parts of the animal. 

 Nerves contain protoplasm in which this power of transmission is 

 powerfully developed. 



We have seen that at some period in the life of all animals 

 when food is abundant, more living matter is formed 

 than is broken down; in a word, that the animal 

 increases in size, grows. But whereas volume increases proportion- 

 ately to the cube of the length (or breadth), surface increases only 

 proportionately to the square of the same dimension. Hence the 

 amount of volume per unit of surface continually decreases as size 

 increases, and thus the chemical action between the internal portions 

 of the protoplasm and the surrounding medium, which can only 

 go on through the surface, is slowed down; in other words, the 

 activity of growth is checked and when a certain size is reached 

 waste becomes equal to repair. At this stage there is a tendency 

 for the protoplasm to divide into two or more pieces of smaller size. 

 This division into smaller pieces is called reproduction, and it 

 is a necessary result of growth. When an animal divides into two 

 equal portions, the process is called fission, but when one portion 

 is .very much smaller than the other, the process is known as 

 gemmation; the smaller portion is called the germ, and the larger 

 the parent, since the latter is somewhat illogically regarded as 

 identical with the original animal before division. A germ very 

 rarely resembles the parent; usually it has to undergo a series of 

 changes during growth by which it at last attains the shape of the 

 animal which gave rise to it; this series of changes in shape and 

 size is known as development. 



Reproduction in the higher animals is closely associated with 



another process called conjugation or sexual 



union. This process consists in the coalescence with 



one another of two portions of living matter. Conjugation probably 



