The purpose of this chapter is to recognize a public obligation to employees in the public service whose duties expose them to more than ordinary risks and to provide a means whereby such public employees who become incapacitated in the performance of their duties or by age may be replaced by more capable employees, to the betterment of the public service without prejudice and without inflicting a hardship upon the employees removed. The retirement pension and benefits provided by this chapter are granted as an additional element of compensation for future services.