CHAPTEK Y. 



THE BIOLOGY OF AN ANIMAL (Continued). 



The Earthworm. 

 KEPRODUCTION. EMBRYOLOGY. 



Reproduction. The life of every organic species runs in 

 regularly recurring cycles, for every individual life has its limit. 

 In youth the constructive processes preponderate over the de- 

 structive and the organism grows. The normal adult attains a 

 state of apparent physiological balance in which the processes of 

 waste and repair are approximately equal. Sooner or later, 

 however, this balance is disturbed. Even though the organism 

 escapes every injury or special disease the constructive process 

 falls behind the destructive, old age ensues, and the individual 

 dies from sheer inability to live. Why the vital machine should 

 thus wear out is a mystery, but that it has a definite cause and 

 meaning is indicated by the familiar fact that the span of natural 

 life varies with the species ; man lives longer than the dog, the 

 elephant longer than man. 



It is a wonderful fact that living things have the power to 

 detach from themselves portions or fragments of their own 

 bodies endowed with fresh powers of growth and development 

 and capable of running through the same cycle as the parent. 

 There is therefore an unbroken material (protoplasmic) continuity 

 from one generation to another, that forms the physical basis of 

 inheritance, and upon which the integrity of the species depends. 

 As far as known, living things never arise save through this 

 process; in other words every mass of existing protoplasm is 

 the last link in an unbroken chain that extends backward in the 

 past to the first origin of life. 



The detached portions of the parent that are to give rise to 

 offspring are sometimes masses of cells, as in the separation of 

 branches or buds among plants, but more commonly they are single 



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