“It is much more difficult to see how it will ever be possible to abandon a system of provision for the aged under which each generation, by paying for the needs of the preceding one, acquires a similar claim to support by the next. It would almost seem as if such a system, once introduced, would have to be continued in perpetuity or allowed to collapse entirely. The introduction of such a system therefore puts a strait jacket on evolution and places on society a steadily growing burden from which it will in all probability again and again attempt to extricate itself by inflation.”