Anything which has internal conditions in equilibrium with external conditions is dead. For example, most plants are in some way phototropic or light seeking. Bacteria are attracted by some chemicals and repelled by others. Female fireflies flash response signals to the flashes of male fireflies. Human beings respond to symbolic communications of ideas contained in words transmitted in written or oral form.
Reproduction is one of the most important definitive characteristics of living things.
Many different methods are used by different species to reproduce, including seeds, eggs, spores, and cellular division. By these means living things are able to bring additional members of their species into the world. Repair mechanisms refers to the amazing abilities of every species to repair or replace structures which decompose, break or wear out. Adaptability refers to the ability of every species of organism to vary genetically under the influence of the environment so as to maintain the viability of the species. Survival is a purpose common to all that lives, and a part of this is an aggressive tendency to multiply and occupy new territory.