ABSTRACT
The line of demarcation between the living and the non-living is notoriously difficult to draw. There are entities in the borderline area which defy classification either way and there are properties seemingly characteristic of living matter which are also displayed by some non-living things. Of the four typical characteristics of life none is wholly lacking in the non-living. First, specific bodily form and structure is equally characteristic of living beings and of crystals, as well as of other physical entities. Secondly, organisms grow and develop; but crystals also grow and often diversify themselves, like ice on the winter window pane, in intricately patterned ramifications. Thirdly, life is distinguished by metabolic change, constant flow of energy and chemical transformation of matter within the stable form of the living creature, but this is paralleled in the candle flame and possibly in more complicated ‘open’ chemical systems. Reproduction, the fourth typical property of life, though more difficult to attribute to nonliving substances, is not altogether lacking in the inorganic sphere and is certainly a property of viruses, the classification of which as living has been justifiably questioned.
