Precise definitions of life, especially when dealing with viruses and other microorganisms are often difficult. Although there is a wide variety of life on Earth, especially with regard to outward structure and form (morphology), all living organisms, both plant and animal, share a few fundamental functional characteristics that together define life processes.

The long-term survival of any species is, of course, dependent upon the ability to reproduce. Although selected individuals and population groups may be temporarily prevented from reproduction or chose not to reproduce, the capacity to reproduce--and to produce subsequent generations that are also capable of reproduction--is essential to the survival of every species.

In some form, all living things show an ability respond to environmental stimulation. The response may result in gross movement, but a response may also be reflected only in a subtle, internal change in the...